There really is no rule of thumb. The key is to determine what your end goals are.
Are you getting your hair cut to maintain a specific hairstyle such as long layers or a long bob? Or are you getting your hair cut to eliminate damaged ends and sections?
If you are getting your hair cut to maintain a specific hairstyle, you would need to go in for a trim every 4-6 weeks depending upon how fast your hair grows.
If you are getting your hair cut to remove damaged tresses, you probably only need to go in every 12 weeks or so.
Ultimately you should discuss your hair goals (maintain a style or remove damage) with your hairstylist and let them give you their suggestions.
Everyone from hairstylists to hair consumers have their own opinions and theories about the best schedule & reasons for cutting your hair.