With Jim the Naked Guy, it was all the time, rain or shine, below freezing or above. As long as it was in his room. With Sara the Naked Lady, it was all the time, anywhere, with an especial fondness for greeting the mailman or unsuspecting co-oppers' parents. (No, actually, the Naked People didn't like each other much.) With the rest of us (ahem), it was when it was really hot or after a sauna (we had a sauna) (this was the 90's, not the 70's, if you can believe that.) Or skinny-dipping. A fair amount of skinny-dipping. The frat boys loved us.
Policy-wise, it just meant if you wanted to get naked, get naked. I think I've told this story before, but my best friend met E.G. for the first time when he was standing in his doorway (he lived at the co-op too) chatting amicably with Sara the Naked Lady. My best friend was rather taken aback, but I was steeped thoroughly enough in the co-op ethos that I barely noticed. (Sara was naked, yes, but she wasn't exactly hot.)