I'm using Ilford multigrade paper developer. Ilford's data sheet says working solution (1:9) will last 24 hours in a capped bottle. I am finding it lasting 1-2 weeks easily. At least I am testing it with some exposed paper and the paper goes full black no problem. As the capacity is 100 sheets (8x10)/litre I am loathe to throw my half litre working mix away after each session of a few prints. What are others' experiences? I don't want to compromise the "learning curve" I am on by adding in too much of a variable of gone-off developer, but I am just not seeing a difference between freshly mixed and 1 week old?