Well, you claim one or two. Then you have to use them both within 24 hours. But um, I suppose maybe I can change the rules or something.
I'll set it to two avatars within 3 days, since not all of us change our avatar twice within 24 hours. =X
So, you can claim two at a time - with a 3 days limit. If you haven't used any within 3 days, then they're free again. If you claim two and only use one within 3 days, then the one you don't use is free.
Don't forget that you can use two at once, one for your avatar and one for your profile picture.
Random tip: If you take two but don't wanna change to the other one within 3 days, you can let it "rest" as your profile picture in the mean time, for as long as you want. Because as long as you use it, it's yours~
☆ 3DS: 3883-6119-6754