Test. I've used it as a tank developer, replenished, and it's lasted for months, maybe even a year, but I've also had it crash unexpectedly, most likely due to heat exposure, I think.
I first experienced this after going away for a few weeks and running six 4x5" negs in my Acufine tank and having them all come out blank. I tested with a film leader, and the developer was dead. I later learned that while we were traveling, the air conditioning in our building went out for three days, so I suspect heat as the cause.