You shouldn't have to replace it very often - once a year - maybe longer.
The older models had a spin-on cover with a latch behind it which was pretty hard to get to. Use a knife blade or a small screwdriver to lift the little latch and then spin the cover off. The newer tubs have two stainless steel screws clearly visible and easily removable.
The screen helps the pumps to live longer, and helps a bit with water clarity. It can usually be left in place and simply hosed off when the water is changed. If yours is loading up, you might want to try to keep the contaminants out of the tub if possible.
