Teeth should be checked at least once a year. Some horses can go longer in between floatings and some need them more often, just depending on the conformation of their jaws---just like people they can have teeth that don't meet up correctly and this can lead to more uneven wear. Just feeling with your finger won't tell you if you need your horse's teeth done. The teeth extend way back into the jaw and unless you can stick your hand back in there up to the middle of your forearm, you are going to be missing some of them when you try to check. You also get only a part of the picture by checking the outside edges of the teeth as hooks often form on the "inside" or "tongue side" of the teeth. This is why you should have your horse's teeth checked by a vet who uses a speculum which allows for a better look and feel of the premolars and molars.
It is fairly common practice to float teeth at the same time as doing vaccinations and drawing blood for testing.