We make fresh Mother Cultures every day. I used to test pH, but now we just go by consistency. What we are looking for is a little bit of coagulation, like a medium thin yogurt. That is where the bacteria will be at their optimum efficiency. We let mesos incubate at room temp overnight. We do thermos at 110-115 for about 6 hours. Beyond these limits the bacteria will produce too much acid and they will start to die in their own acidic waste. That will be very noticeable with the whey starting to separate. If that happens, the culture is still very viable, but not optimal. Less bacteria in the beginning means incubating a little longer, but you also stretch out your dried cultures. We use just 1/16th tsp for a gallon of skim.
Frozen MC cubes really don't go bad, but Mother Cultures are cheap & easy to make, so I wouldn't keep them more than 6 months. Fresh means that the bacteria are awake, reproducing, and ready for a quick lactose meal. It's easy to make just 12-16 ounces at a time, so just make enough to fill one ice tray. Better yet, make it fresh every time. You will use 1.5-2% MC, so around 4 ounces for a 2 gallon make. If you are doing the cube thing, measure how much goes into the cube tray and you can deduce how many cubes to use for each make.