I don't replenish either. When mine is dead then the whole print is sort of faded. It's a hard cutoff from working okay to dead. The main thing I notice is that during the life of the developer the print needs a touch more exposure.
What are you topping up the developer with? More developer? Kodak likely lists a number for square meters of paper per so much developer. Depending on how much you're topping up then you might be replenishing. Are you pre-washing?
With me I find the bigger issue isn't the number of prints so much but the amount of time since I mixed up the developer. I can get quite a bit more then the kit I'm using claims I should if I use the stuff quickly.