Most print developers have a very short working strength life. They are very active developers in general, and they get a lot of oxygen mixed in when used in trays for print processing. The act of agitating a print oxidizes the developer. If you are continuously processing over several days and you have a big volume of developer in your tray (say 1/2 gallon, or so) then you can "get by" with spiking the developer with some fresh, but the developer will start turning brown, making it hard to evaluate the print. This is general information for all print developers. The fact that print developers need to be very active developers means that they cannot have the amount of restrainers and preservatives in them that film developers can. These restrainers and preservatives tend to "slow down" oxidation.