After previous expansions launched, how long was it before prior xpacs contents exp requirements were nerfed?
This was already answered, but to confirm it, patch 4.3 was the patch that lowered the XP needed to level through Northrend and improved dungeon quest flow for both Outland and Northrend dungeons. I'm not sure which patch lowered the XP needed to level in Outland, although this forum thread on Wowhead seems to indicate it was in place by November of 2008. That would mean Patch 3.0.3, the major content patch for Wrath of the Lich King, or slightly before that.
What we can take from this is that Blizzard doesn't have a set pattern for nerfing old content's XP requirements. The devs do it when they think it's warranted, no sooner, no later.