I used to take one side of the tyre off the rim and clean inside . Over time the sealant hardens and sticks to the rim . If you get lots of punctures and ride a lot then of course you'll use up more sealant so depends how much you ride and depends how much you put in to begin with . Also in winter time because of the cold it speeds up the hardening process of the sealant .
So only put in as much sealant relative to where and how much you ride .