In regard to how often you change your grips depends on how often you play and practice. If you are a very active player your grips will wear out faster. What you want to pay attention to is if you are getting wear marks or if your grips are becoming slick. The first thing I tell people is to wash your grips with hot soapy water. If they are relatively new this should bring the tackiness back. If they are still slick after cleaning, then you should replace your grips.
I believe in having light grip pressure on the club. If your grips are slick you will have to squeeze harder to keep them in your hands and in my opinion this will hamper your swing.
What I have told people to do in the past is to find a popular grip that you like and put them on all of your clubs. If the grip is popular the manufactures will tend to leave them in the lineup for a long time. Because of this you can then just replace the grips on your favorite clubs that you use the most which will wear out faster.
This will help keep your cost down since you don’t have to have every club re-gripped at the same time.