I've mixed up 2x 500mL using both the liquid Tetenal kit and the powder Unicolor kit. In fact, I'm on the second half of my Unicolor kit right now. I stored those powders in the original foil bags (after I'd measured out half) and bagged those foil bags in plastic Ziplock type bags. I think they were probably sitting like that for about a month, then I mixed them and they've been going for at least 10 rolls so far, I'm about to do 2 more tonight then toss the chems.
That said, I'm just going to mix up the full liter of the new Unicolor kit I just got. I don't think it's worth the trouble dividing it, since I've been going through about maybe 6 rolls of color a month? So if I could make the entire liter last for 2 months and do 12-15 rolls I'd feel satisfied.
Oh, MAJOR caveat here: When you measure out the powdered chems, MAKE SURE THERE IS PLENTY AND I MEAN PLENTY OF VENTILATION. I did not know to do this last time and I think I felt my life get shorter by a decade. Once those powders hit the air they're no joke!