This is a great question and ultimately it is your choice as the owner. However, I can do my best to explain to you what the benefits are of a regular bodywork schedule.
A one-off massage session is a great treat for your horse. I'm sure he or she will also feel pretty good for a little while after. Unfortunately though, a one-off session isn't the most productive for most horses.
We assess the horse initially, and we work holistically to solve imbalances, tension, and posture issues, among other things. Often it takes more than one session to unwind what is potentially years worth of compensation and asymmetry. If we wait too long between sessions, it can become an even longer process. Rome wasn't built in a day and the body cannot change it's posture permanently overnight (although it is always super awesome to see big changes in a single session!).
Doing regular bodywork is also hugely beneficial for keeping your horse's muscles and other soft tissues in prime condition, so you are less likely to run into preventable injuries and setbacks. Your equine therapist is also trained to notice subtle changes in the horse's body and could save you a lot of headache down the road by catching an issue while it is just beginning, that perhaps would have taken a lot longer to notice otherwise.
We all know we aren't supposed to leave our horse's feet untrimmed until they're ridiculously long and the horse is sore before we call out the farrier. Try to look at bodywork the same way. Utilize bodywork as a highly effective form of maintenance and preventative care for your horse.
As a guideline, I generally suggest massage sessions are booked once a month. This has proven to be a very effective maintenance schedule for the majority of horses. If we are tackling a specific issue, sometimes I will recommend a couple of sessions closer together, to minimize the amount of time it will take to work through the problem and prevent a backslide.
Some of my clients are on a 6 or 8 week rotation, and that works fine for some horses as a maintenance schedule if they aren't working extremely hard and aren't battling any major imbalances or conformation challenges. If we notice a potential problem starting, we can always shorten the time between sessions temporarily until we get the issue solved.
Even on a monthly massage session schedule, you're spending less than $1000 a YEAR on bodywork - money very well spent to keep your horse performing his or her best, and helping to prevent injury, time off, and potentially much larger bills down the road.
Please feel free to comment or send me a message if you have any questions!