If you attach your press ons with adhesive stickers, you can use them until they no longer look nice to you. However, before using them again, make sure that no adhesive residue is left on the underside and that all oil residue is removed.
If you attach your press ons with nail glue, you will most likely only be able to use them once, as a remover is required to remove them, which will probably change the appearance of your press ons. The adhesive residue is also very difficult to remove.