I made up 1L of developer near the end of June from the 5L kit and used half of its capacity (7 of 16 rolls). It's been in the fridge at 2C in a PETE bottle topped off with butane for four months now; this afternoon I ran a test roll through it that I exposed with two other identical rolls in June - and it looks identical. So as far as I can tell, the C41 developer keeps happily for four months with no degradation if you treat it right. I don't have a densitometer or test strips to verify that claim with huge accuracy but when I hold the negs up to the light next to each other, they look identical - I can see no colour or density differences by the naked eye. Of course, it scans as well as the rolls developed 4 months ago but I don't have my RA4 up and working yet.