There is a limit on how often you can reuse cooking oil. Like most things in this racket, what that is "all depends." It depends mostly on the type of oil, how high it was heated, and for how long. The purpose of filtering is to get rid of any leftover bits of whatever you cooked, so they don't affect the next batch of food, or burn when reheated. Filtering has it's limits though. It won't make oil not taste like fish. After cooking something with a strong taste or aroma, it's useful to fry-off a couple of pieces of white bread before reusing to purify taste and aroma before cooking anything else. Potato also works, but not as well.
When oil is overused, the water and the fats begin to separate. The color darkens, it develops a stale odor and taste which attach to the product. Product stays greasy even when well drained. If you notice any of these things, discard the oil immediately. Immediately as in right now, not as in just one more batch. Next, the smoke point drops radically, and the oil produces some very unhealthy compounds at normal cooking temperatures that attach to the food and enter the air.