Let's see. My longest climb would be up Powder River Pass in Wyoming. I believe my starting position for the day was at 2000ft, did who know how many feet to the base of the climb, then rode the roughly 25 mile long climb up to almost 10,000ft.
Of course, after reaching the top, I still remember there being one 5 mile climb, and a few 2 mile climbs. That was a hard 100 mile day.