How long can you use a stop-bath?

A few years ago I’ve started using a stop-bath in my black-and-white film development process, to speed up the process, and reduce wasting a lot of water.
I bought a bottle of Fomacitro stop-bath, that says it has an indicator component, it will change its colour once it gets exhausted, from orange-yellow to blue-green. But, I’ve never used a stop-bath before, I’ve never seen that colour shift, how would I know it is getting exhausted?

Apparently, that switch isn’t as abrupt as I was hoping it would be! I mixed a litre in May 2020., and just kept using it. The last time I developed a tank, I realized I might have seen that colour change – it was looking pale. After checking my notes, I’ve realized I’ve used it in the process of developing fifty rolls of film, apparently!

I decided to mix the new batch, just in case, and to my astonishment, freshly mixed stop-bath is significantly more orange than the one I was using for the last few months, indicating it was, indeed, getting increasingly more exhausted!

What have I learned? Fomacitro will, indeed, indicate it is getting exhausted by getting progressively more transparent, but that progression is very gradual. Using it for fifty rolls might be just a bit pushing it too much!

Fresh Fomacitro stop bath