Developers mixed to working dilution tend to die after a day or two due to the oxygen dissolved in the water that you dilute them with. How dead it is will depend a lot on the details of your water (oxygenation, pH, presence of iron and/or other oxidising agents) and the developer (again, I don't know about Tmax). For example, D76 mixed 1+1 I wouldn't be much worried about at all, but Rodinal at 1+100 I would discard after a few hours at most.