Some folks train steady year round and don't really take an FTP hit, but if you've had a busy season and expect to do a full winter of base building and FTP development before next season then conventional wisdom would tell you to take at least some downtime before starting the cycle over again.
I typically take up to a month off in the fall, I'll still ride and do other outdoor stuff including long hikes but I stay away from any structured intervals and let life and family take priority over training. If I'm riding and feel like going harder up a hill or just rolling fast I don't hold myself back, but I don't push through discomfort or stay on a strict schedule of any sort during my downtime.
Yeah, FTP or at least achievable long interval power typically takes a hit after the downtime. Whether FTP has really suffered or whether it's classic blocked up legs from too much rest is hard to know but my power numbers drop when I get away from structured training.
The good news is that over the last three (going on four) seasons of riding with power I return to my previous high points very quickly during winter and preseason training. I don't always see it when I'm stuck indoors on the trainer, but it seems to reliably come back and climb from my previous highpoint.
For instance from a 2006 highpoint of ~ 260 watts for 20 minutes I hit outdoor training in April of 2007 with 20 minute intervals in the 280 watt range and didn't hit 300 watts for that duration till July. In 2008 I could hold 300 watts for 20 minutes by May but topped out at ~ 310 watts by midseason. This past season I managed a few indoor workouts with 20 minute efforts up beyond 300 watts in early March, improved on my previous 310 record in May and managed to break 320 watts for the same duration a number of times in June and July.
The point is that my sustainable power does take a hit with the post season downtime, but the power comes back and at least for the past three seasons has come back stronger and earlier each year relative to the previous year. This season I'm racing cross so I took some easy weeks after wrapping my road season in August and will have to figure out how much rest to take when cross season finishes before getting into structured road prep. It'll be interesting to see whether I come out ahead or behind next season with all this late season intensity, but cross sure is fun.
Anyway, there are definitely folks who train year round with great success and don't burn out but most folks take a well earned breather after a busy season, especially if they intend to do a full winter build cycle. But the downtime doesn't have to cost you FTP in the long run.
Good luck,
-Dave
