I used the Tetenal Colortec 1l-kit for my first selfmade C-41. On the box they tell you that there is a maximum of 10-16 films
(135) that can be developed. I used it for 29 films with no noticeable (for me!) falloff in quality. The working solution was used from 10th september until 30th october. It was stored in my bathroom, I did not squeeze the air out of the bottles!
The pictures were taken with the Pentax PC35AF: