A properly designed belt drive should have great life. If you are experiencing short belt life you may have damaged drives or alignment issues - if the drive was designed properly in the first place.
An HTD belt drive is fairly forgiving, and will deal with dirt, oil and occasional overload. The timing belt in the average car goes 60,000 miles without issue, and many owners ignore the scheduled replacement - I have seen many with 100k + miles on them.
Without knowing the drive sheave tooth counts I can't do all of the calculations for you. But the belt part number posted here (327 tooth) is a non-stock part. It is a 9mm wide belt.
A 5mm pitch HTD belt, 9mm wide can carry anywhere from 2 to 12 Un-Corrected horsepower @ 6000 rpm, depending on the tooth count of the smaller sprocket.
The power correction is a step that is often overlooked. A single cylinder piston engine has one power pulse every 720 degrees, and a minimum correction factor of 2.0 should be applied. With any type of oscillating load, an additional correction of 0.5 or more should be added.
If you have a 20 tooth sheave on the engine (driver), it will have short belt life with a 9mm wide belt. If you have a 36 tooth sheave, the belt will last 2,000 hours or more.
So what are the tooth counts in this drive?