I remember the year I spent in Icecrown Citadel. I'm not really exaggerating - it was from December to December, so about a year total. It was about the longest time I spent on a raid, including the days of Molten Core - for comparison, Molten Core was the only real endgame raid besides Onyxia's Lair from November of 2004, WoW's release date, until July of 2005, so roughly eight months. Interestingly, the Shadow of the Necropolis patch (patch 1.11) came out in June of 2006, so in the year between the first and last raids of classic WoW we saw MC, Onyxia, BWL, Zul Gurub, Ruins of Ahn'Qiraj, Temple of Ahn'Qiraj and finally Naxxramas. All of these raids released between July of 2005 and June 2006. Not all of these raids were replacements for previous ones - Blackwing Lair and AQ 40 were considered 'sidegrades' from each other, at least until one killed C'thun, who until the release of Naxxramas had the best gear in the game. The two 20 man raids, ZG and AQ20, did not replace BWL or evn MC gear, they just provided another place to go.
Because of the way raids were structured back then it's a little misleading to compare classic's raid release schedule with our modern one. Raids were something a very few players overall did - there was no parity between smaller and larger raid sizes, no LFR, no flex (although by the time Naxxramas came out, some guilds were running MC, Onyxia and even BWL/AQ with smaller raids to maximize gear acquisition before heading into Naxx) and the only way to gear up for raids was either to be carried through said raids by geared groups and handed all the stuff they didn't want or need anymore, or to start on the ground floor and run the level 60 dungeons. The design wasn't structured around raiding being accessible or allowing a larger group of players to see these fights - raiders got to see them, and if that was 10% of the people playing the game, that's what it was.
It's interesting to look at how players react to raid content now. A commonly expressed sentiment is that Throne of Thunder, a raid first released on March 5th, 2013, has been around too long and players are eager for new content. This is a raid that has been around for six month, and will be superseded around the time it enters it's seventh. While hardly the shortest time a raid has ever had to be run through, it's not much longer than the initial tier of Mists raid content, either. Mists of Pandaria released on September 25th, 2012, meaning that from October 2012 to March 5th 2013 we only had MSV, HoF and ToES - a time of about five months. What makes five months acceptable and seven months unacceptable? Are two months that much longer to raid a zone?
I'm not drawing a conclusion here, because there are a lot of variables to consider. Looking back at The Burning Crusade, the first expansion for WoW, one of the things that comes to mind is the sheer number of raids that released. BC launched with three raids, introduced two more tiers with two raids a piece, then a stand alone 10 man raid, and followed that with a final capstone raid with better gear than the last tier. This is a surprisingly high ten raids in one expansion. Wrath had three raids to start, then a single raid in Ulduar, a single raid in Trial of the Crusader, and finally ICC with a last sidegrade raid in Ruby Sanctum for seven total. It took each of these expansions about two years to release all of their content. Some of the content (like Ulduar) didn't get a lot of time before being upstaged by a newer raid - Secrets of Ulduar patch 3.1 released in April 2009, while Call of the Crusade patch 3.2 was released in August of that same year, meaning that Ulduar didn't even last the whole summer of its release, a mere five months total. Patch 3.3, meanwhile, released on December 8th, 2009, which means that the Cataclysm release date of December 7th, 2010 marked a year of running ICC (and six months of running Ruby Sanctum). '
Ruby Sanctum didn't really 'count' in many players' eyes - the gear wasn't really better than ICC and there wasn't much story to the new raid which was similar to the Sartharion fight in its use of minibosses before the main event. Cataclysm, for its part, mirrored the Wrath release cycle but generally shortened the time that its last raid lingered, ten months for Dragon Soul vs. 12 for ICC. There was also no added 'teaser raid' like Ruby Sanctum. Both Cataclysm's introductory raids of Bastion of Twilight, Blackwing Descent and Throne of the Four Winds and its mid-expansion Firelands raid (patch 4.2) lasted roughly six months before being replaced, meaning that in general Cataclysm had the most reliable raid release schedule of any expansion. Yet I clearly remember complaints that both the Firelands and Dragon Soul were out too long. This again brings me to wonder what that means in the modern raid release cycle? Six months into Dragon Soul, some guilds were working on finishing heroic Madness of Deathwing, some guilds had already finished it and still others never actually killed normal Madness, even with the raid-wide progressive buff.
I would submit that often the community seems to reach a consensus for 'too long' based on when the most progressed guilds in the world finish with content, and ignores that the vast majority of raiders (who play in raids far, far more accessible than the vanilla or BC or even Wrath raids ever were) simply do not kill these bosses that quickly. But the other complication is the rise of LFR difficulty, which in many ways has replaced what was a major portion of the game for several expansion, namely chain-running heroic five man dungeons. LFR has essentially become the content that a great many players consume, and as a result although the fights are tuned differently, the experience of those fights has become something players who never even attempt a 10 or 25 man normal raid has each and every week. In essence, the shelf life of a raid, its freshness if you'll allow me to steal a metaphor, is not preserved because the accessibility means that more people can feel like they've seen a raid enough times. If the only time you got to see Icecrown Citadel was in a PuG in September of 2010, how could you get sick of it? Groups that raided it for a year solid sure could, but now, we can all raid things solid for months at a time.
This isn't me decrying LFR or its effect on the game, by the way - I'm far from convinced that the game was better when a huge chunk of its content was kept out of the hands of the players. But I do think we need to consider our expectations for new content in this new era of accessibility. Raid design takes time and effort - expecting a new raid every six months may be pushing it. Even if you feel like you've run Throne of Thunder enough times already, a new raid isn't just waiting for some fruit to ripen and fall of the vine, there's labor and effort in its design and execution to consider. Throne was on the PTR for three months before it launched, and was clearly in design for significantly longer than that, and Siege of Orgrimmar is currently on the PTR right now and has been for months. As raiding becomes mass content, it's still going to take a while between raids. If seven months is too long, then you'll often end up waiting too long.