I replenish bleach and fix for C-41. After each batch I remove so much of the old solution. Exact amount varies with the amount of film and the chemicals used. IIRC my bleach is considered low replenishment so very little bleach gets replaced every batch. OTOH I remove more fixer per batch. I then replace that amount with replenisher. You'd do something similar with paper but you'd keep track of the amount of paper instead of the amount of film.