It strongly depends on your motive, intergration of the experience, and how your time is used between 2 trips.
If you have a few weeks off, can do your own thing, sleep when you feel you need to, not having to go to work, doing a lot of exercise, have your stress level at it's lowest point, spended time with beloved ones/friends between the 2 trips... one can do another strong trip after 1 week.
One of the trickiest parts is that, you may ingest shrooms yet again when your mind is still intergrating the previous experience without noticing it yourself when sober. But once tripping, totally part of the current experience.
I'd have to say one month. After an excessive dosage of mescaline, the period off for me is about 6 months. Dunno why. And personally, I love shrooms during the winter, I'm only doing acid on a crystal clear summer day.