Whoa whoa whoa...I feel a need to explain myself here.:-) Because while it's true I purge regularly with no regrets, I'd never purge on the basis of not having worn something for 3 months. Because, as others said, that would just mean purging stuff that was out of season!
No...the way I purge is more like this: Big purge at the end of a season. Anything that's worse for wear, or no longer fits my style direction or lifestyle, or anything I'm just plain sick of -- out. Then, at the beginning of a season, I try on everything that I'd put away for safekeeping, decide if it still fits, still works, etc. And if it no longer works, out it goes.
And then there are the mini- in progress purges. Within a season. But those are for items that bug me (usually it's a fit issue unknown until now.) Or items I'm not wearing because I upgraded and found the perfect replacement, which I far prefer. Or, you know, the underwear gets shabby and I decide to update. That kind of thing.
I also relate to what Janet said. When I shop and buy something, I tend to reconsider all the items in that category in the closet and get rid of those that no longer seem as fab.
I don't consciously do the 33 thing for each season, but it turns out that I do include about 30-40 items per season in the closet -- not counting gear, layering Ts, footwear, scarves, belts, or outerwear coats etc. Just articles of indoor clothing. More like 30 for the shorter summer season. More like 40 or 45 for the longer winter season. This feels like a pretty good number. I always know what I've got.
Oh -- and I make my "seasons" more like six, vs. four. Obviously, there's overlap in clothing items. (So most of my jeans cross all seasons, although truthfully I almost never wear jeans in summer due to the heat). But some items are only for one or two seasons. And when I put those away, I bring out the new (seasonally appropriate) replacement.