Dear Malin,

My answer is mainly consider the time you need to detect pH changing if you are using a pH meter.

In my experience working with pH probes, the time required for reading or stabilizing of pH could be quite different. But a practical approach is, when reading values reaches a low variation, you can accept that as the pH of your system.

Hope it would be useful.

The amount of time it takes to change pH level depend on the substance used and the starting pH level. but the process usually takes one to two years.

This is an question for a chem engineer which I'm not. My guess is that it will depend not just on the flow rate but on the efficiency of contact.