This can depend a lot on a few different factors. The first is how often you’re using the abrasive, the second is what kind of abrasive you’re using, and another is the quality of the abrasive in question. Damaged abrasives will often lead to damaged wood. Instead of giving your piece and nice finish, they’re more likely to cause blemishes or damage. This is why you’ll need to frequently change abrasives after using them for a long time period. You can check to see if your abrasive is no longer good by taking a look at how dull or sharp it still is. You do this via sight, or by softly gliding your fingers over the top of it. Once you get more used to using a sanding drum, it will be easier to know just with a single look.