I've mixed a stock solution several times. Almost all of my darkroom work is on the weekends, and on one occasion, I mixed the stock solution, used some one weekend, and refrigerated the stock solution until the next. It didn't work - even if I developed Azo for three minutes, I got almost no image.

However, if I mix a stock solution and use part one night and the rest the next, it works fine. I notice no difference between prints developed on the two days.