All good. Shouldn’t rely on a timeframe anyway but more when they stop being effective. Having multiple forms of smell control also helps if smells a big issue, so your not throwing a the the chickens in one basket.
Absolutely, like so many things, there’s variables. Like how often and for how long they get run in higher humidity levels over say 65%. If it’s constant or timed use etc..
As an example I’m a sealed room and during lights on the fans only come on for 5 mins every 2 hours, run for about 10mins at lights off, and then 5 mins every 3 hours during lights off. The lights off timing gets changed for growth stage / how much foliage their bearing. I also run higher temps n humidity when injecting CO2. So like you said...no set time n use my judgment on replacing.
Yep, if I’m running a high smelling strain I’ll run a carbon scrubber as well.
For the cost of a filter, if it were on a fan running 24/7 in all conditions, I’d probably change it out every 12 months. That’s just me.