Honestly, riders at my barn tend to only give baths if they are going to a horse show or if their horse is really dirty. However, during the summer, I rinse pretty much every horse I ride off with water! Bathing a horse too much can be damaging to their fur.
Depending the aim of the bath. To clean him: when necesary. To get sweat off: when the horse is still warm. Make sure you do not bathe him to often.
Wash them with water every time you ride but don’t give them a bath often because it strips the oils from their fur. I would say give them a bath at most once a month. :)
I wash my horse with water after each training when it’s hot. If it’s not very hot just under the saddle and legs. Before the show I take a shampoo and give her a nice bath 🛀 to make her shine! 😂
Rinsing your horse with just water will not affect them at all. But if you are giving them a full bath then it will. Bathing them strips away oils from their coat. Because of this I wouldn’t bathe them often. Usually just water will be fine, but I understand that a deep cleansing is good every so often. So I guess the answer to your question would be (for me) at most every 2 weeks. I hope this helps!