Use as much solution as you can. When I tray develop 8 x 10 sheet film I pour in the whole gallon to develop two sheets. For rollfilm I use about a quart of solution in a tank where it only takes about 16 oz. to cover the film. If I had a bigger tank I'd use it. About every 8 sheets or 8 rolls of 120 or 32 sheets of 4 x 5 I replenish with 6 oz. of developer from the second gallon. I mixed up this batch last year when there was snow on the ground (around the end of November, I think) and it's still going strong. I've still got about 3/4 of a gallon of replenisher.
After a dozen rolls or so it begins to look like the poster child for a toxic waste dump. Don't worry. It'll produce negatives capable of breathtaking prints no matter how funky it looks. I think mine must be supporting numerous ecosystems at this point.