My first 4E campaign went from June 2008 (right when 4E came out) to April of this year. We ended when everyone had just hit 14th level. Paragon tier was slowing things to a crawl--with the high hit point totals of monsters and a plethora of in-play options for characters (particularly immediate interrupts), combats were averaging about an hour and a half. We all agreed that paragon tier was just too slow and started a new campaign at heroic tier. For the foreseeable future, we'll be playing at heroic tier exclusively.