They always go bad eventually. It depends on their quality (quality of materials used. The best use full leather covers, and cork & rubber centers.), conditions such as wetness, and how often and how hard they are hit. I find that a bucket of practice balls seem to loose a noticeable amount of their C.O.R. ( coefficient of restitution ) after being used for batting practice 2-3 times per week for 1 year. Coefficient of restitution, often referred to as "pop", is a material's ability to store and release energy that is applied to it. This is O.K. in a batting cage but on a field it's nice to see how well the balls are coming off the bats. When they lose their C.O.R., they have lower exit speeds and therefore don't travel as far. You will usually be able to tell if baseballs are going bad by listening for a bad sound when the ball is hit, or by examining the ball for any irregular shaped areas, or soft spots and loose fitting leather covers.
Hope this helps.
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