Yes, bottom paint can go bad. Bottom paint can last several years in the can provided it was unopened or sealed properly. However, high copper content bottom paint that has been in your garage for a year or more will require a mechanical mixer because of the settling of heavy copper in the bottom paint.
The actual lifespan of bottom paint depends on the type of bottom paint used and the environment in which it is used. Hard bottom paints do not release or wear off of the boat. Therefore, after about 12-18 months maximum the copper in the bottom paint will oxidize and become ineffective. In addition, if you leave the boat out of the water for 30 days at anytime during the life cycle, the hard bottom paint will oxidize and will become ineffective. For optimal protection, hard bottom paint should be applied once a year. Since hard bottom paint builds up on the surface of the boat bottom after each application, you can typically need to apply one heavy coat each season.
Just like hard bottom paint, ablative bottom paint will also oxidize, However, since it is ablative (wears away) there is always fresh copper or biocides being exposed to keep your bottom paint working. Therefore, ablative bottom paints can last for multiple years. This depends on environmental factors such as, how much you use the boat, wear you dock the boat (fast currents and tides will wear off the ablative bottom paint faster), and how much ablative bottom paint was actually applied.
Generally speaking, when it comes to ablative bottom paint, the number of coats (or thickness of each coat), the longer the bottom paint will last. When using ablative bottom paint you should always apply multiple coats ( at least two heavy coats. A good idea is to used a different color for the 1st coat. When you start to see that color show through – its time to repaint your boat. If you leave your boat out of the water for more than 30 days, ablative bottom paint will still work. It is a good idea to lightly pressure wash your boat, or lightly rub the bottom with a wet old t-shirt to remove the oxidation before relaunching the boat.