I used to split the 1L Tetenal kit into 2x500mL, leaving the other half of each concentrate in the opened containers. It always lasted until I had exhausted the first 500mL and seemed to work perfectly when I diluted it down to working stock. That being said, I did occasionally have batches that mysteriously died, and because I wasn't very scientific about determining the cause I can't 100% rule out the storage of the concentrate.
I've since started using the Unicolor kit, mostly because it's cheaper
At first I weighed out 1/2 the powder to continue using 500mL working solutions, but then I decided that it's pretty stupid to do that and now I just mix up the full liter. Also, I strongly advise EXCELLENT ventilation when opening, measuring, mixing, even looking at the blix powders. I wouldn't be surprised if I took a few years off my life that first time I poured it out in a completely airtight bathroom