How often should filters be replaced in A/C unit ?
It all depends on the surrounding environment but, typically every 30 days but, I don't know anyone who does that. Twice a year is good.
How often should fire springler system be tested ?
Sprinkler systems are not tested, they contain some of the nastiest water you'll encounter and cause alot of damage when they go off. However, fire suppression equipment is an annual inspection.