It totally depends on your phosphate input and levels. If you have anything measurable it's a matter of a couple weeks. Undetectable and low feeding/bioload you could go 90 days. 30 days for a normal bioload is probably conservative but won't hurt.
Just test the effluent coming out of the reactor. If phosphates are 0, it's still good. If they are elevated, either turn down the flow until it reaches 0 again, or replace it.
