Two to three DAYS, and no longer.
in a bread box
Bleach saltbaking sodacoinhair spraysodasugarvinegarlemon juiceflower foodplain water
If you are talking about ones to fix your bite on your braces, change them 4 times a day; after breakfast, after lunch, after dinner, and before you go to bed. Take them off when you are eating meals...
Keep it tightly capped, in a cool, dark area, but not in the refrigerator. A cabinet that is not near the stove is the best place.