The frequency with which a horse needs its teeth floated varies based on the age of the horse and the conformation of its mouth. Some horses are more prone to develop problems with sharp teeth than others. In general, you should have the horse's teeth checked annually. Many people choose to do this when they have the veterinarian out to do spring vaccinations; they also check the horse's teeth to see if floating is necessary. Horses that have a history of dental issues, or older horses, may need to be checked more frequently.
Signs that the horse may be having teeth problems include dropping feed, head tilting (especially when eating), mouth-related resistance to the bit and riding, whole grains left in the fecal material, and odors coming out of the mouth. In addition to floating, horses may need wolf teeth removed. Or young horses may have caps, created when the deciduous tooth does not completely fall away when the permanent tooth comes in, that need to be removed.
If you have any concerns about your horse's teeth, a good first step is to have a veterinarian or an registered equine dental technician examine the mouth for any problems.