It can happen in as low as couple of weeks and a lot depends on what kind of load is having the filter. If it is on the system where it’s only high bio loading, it lasts for two weeks. If it is loaded on the system like downstream of UV light, it could last six months. It depends on where it is in the system, so you just have a filter distribution system perhaps in the loop. Replacing it on a monthly basis is probably a pretty good idea. You can always follow the downstream counts from the filter and determine at what point growth really starts but typically some way between 2 weeks to 1 month is fine. And if it’s heavy contaminated stream flowing for a much less time, then even 2 weeks is fine.