I leave them up for quite a while, then trim most of them and put them in my scrapbook.
But a few years ago, I needed to quickly fill a collage frame that was part of a pair, so used the ones of all the neighborhood kids. I liked it so much I've kept it up ever since. - I make a large photocopy of the old collage before redoing it, so someday I'll bind the copies together and have a "watch-us-grow" book.