i think it depends on you personally. my brother did Seattle to Portland fixed in two days (180 miles or so). he also did a 38 mile race around Bainbridge Island ("The Chilly Hilly") that included 4000 feet of climbs (or more?) in one hour and 15 minutes.
but hey, he's in great shape and rides 50+ miles daily, fixed.
to me, anything over 30 miles is unnecessary, but I often ride up to 30 miles fixed.