My general answer, for any type of machine, is that you never know.
It’s best to order spares ahead of time and have them handy. Just label them so that if you don’t need them for a few years, you can identify them quickly. It costs mobey now, of course, but you’ll thanks yourself if/when you need it.
I’ve had to replace the engine and deck belts on my Ferris zero turn a few times.
Have never had to replace a belt on my tractor.
Had my skidsteer fan belt break on me while snow blowing last winter; first time in the 13 years I’ve owned it. Started searching online for a new one and then remembered that I had bought a spare years ago and stuck it under the seat.