I would agree with Jim's approach to amidol. I weigh and mix as I need to. The general concensus seems to be that Amidol will last about one day of printing. Possibly a bit into the second day. Those who claim that the developer is consistant in it's action beyond that point are few. As I said this is the concensus of opinion. I have not personally verified this myself. I don't know what the chemical reason would be since Amidol is supposedly not subject to effects of it's own oxidation.