Depends, a can half full or less that has been reduced (without hardner) can build pressure enough to pop the lid, the reducer builds pressure more then the paint itself. So long as the paint lid is kept tight it should keep for a long time. There are some exceptions like Dupont (Axalta) that has the tint 801 in it the white tint will lump up and separate and can not be mixed back into the paint it just ruins the formula and the paint color will be off. If there are other makers like PPG etc that do it I'm not sure but do know that Dupont does it.
