Try to play them every day, or at least every other day if you don't have a lot of time.
It's good to know all of them in general so that if you notice a particular aspect of your technique faltering, you can pick up the etude which most closely corresponds to it and be back in shape within a week or two.
I've so far learned Op. 10 No. 1, 9, 12, and Op. 25 No. 6 and 11, and I play them just about every day. When you practice them as warmup pieces, though, be careful not to play any faster than is 100% comfortable, even if that means playing at half or a quarter the suggested tempo.