My normal 28-mile training route has ~900 feet of elevation gain. I consider that pretty flat. The favorite "benchmark" climb in my area gains 1300 feet in elevation over 3.2 miles. When I'm in shape during the season, I'll often do rides that include 2000-3000 feet of elevation gain. I think my longest day was 5200 feet of elevation gain; that was a lot of climbing! The longest climb I've done was around 13 miles at a steady 2-3% grade. A complete lack of shade and record-breaking temps near 100-degrees made that one of the most difficult, too!