3 weeks on 1 week off works great for me.

I always start the first week of every block somewhat conservative, adding 10% volume or intensity each week until the final week, where I really push myself and get it out of me. My week off consists of very little biking. Helps give me a chance to do other activities like hiking, playing basketball etc... No structure, but still active for general health purposes.

By the time the next block starts, Im feeling fresh and motivated. I always start at a slightly higher level than the last block to promote progression. The cycle repeats for a couple months, then I take a full week or two completely off between seasons.