Because there are so many factors at play, it's challenging to provide a definitive response. Some horse owners choose to never bathe their horses, while others choose to do it once or twice a year, and still others up to weekly. However, there is more to bathing than first appears, therefore it's critical to ask yourself a lot of questions that will only affect you - not what other people do.
Is it the peak or off-peak season? If you're in-season, you should look at the schedule for your show or clinic. Wait till right before the event if it's coming up soon so the horse looks his best. You could certainly skip the wash and use a blanket to keep yourself clean if your off-season is during the winter. Shampoo can be used to wash tails as necessary. If you want, you can skip a thorough wash in the winter and instead use hot towels and spot cleaning.
How is the weather today? Unless you have a heated wash stall where your horse can stay warm, it's usually not a good idea to bathe in cold weather. Nobody wants to cope with a cold horse because it might always result in a sick one. Try using a heated, moist towel in a circular motion on the horse's coat to best remove dirt if you don't mind the horse's coat growing dusty in the winter. This will perform similarly to how a Swiffer works on your kitchen floor and will help remove dirt without getting the horse wet.