Very very very very very rarely. Unless they are showing severe wear or cracking we dont change them. We put over 1,800 hours on our Lazer HP before the deck belt finally let go on us. I did replace a drive belt for the hydros because a stick wedged up in it and shreded the belt but that doesnt count because it wasnt wore out. There is only 2 belts on our Lazer. One that drives the hydros and one that drives the blades. Now the way we do it isnt the best way I will admit that. But we carry a spare mower in case one goes down but not everybody can do that. I would say it would be wise to change the belts every other year just to be on the safe side so you dont have to worry about down time.
I carry a set of belts in my truck, just in case. I check them every week when servicing my mower, if one starts to look like it is fraying badly, I will replace it then. The only belts I've had to replace are deck belts on a ZTR. My walk behind gets new belts every year because it is belt driven and wears them out fairly quickly.
My first Dixie went 1200 hours on all of the original belts before I changed them. I only changed them because I was getting nervous since they had that many hours on them. I keep them as backups for my newer Dixie that has 500 hours so far on the original belts.
Had an 03 Toro Z Master had to change the Hydro belt bought every 300 to 400 hours. Looks like this new G3 will need changed less often.
When they break I change them. Usually not too much of a hassle. I have a complete second set for everything with me when I mow. That being said, it doesn't happen often. I hope I don't jinxc myself.
