This is a long disputed question with no hard and fast rules or answers but, consider this:
1975 to 1987 4-cylinder cog belts are a very common automotive type that should and will last safely to 45,000 miles, provided they are used in a car with close ratio automatic shifting and fairly medium to high freeway to highway use. BUT, put them in a Gold Wing with its manual shift and wider gear change steps creates another situation. Bang, clunk, up and down, over and over - lashing and stressing the belts harshly, setting up stress and shear points around the belt.
You can't tell a thing by looking at them as they NEVER show wear, stretch or cracking. The belt construction, a bias fiberglass rubber laminate, does not allow for it because valve timing must NEVER alter.
Hundreds of discussions with customers who have broken belts puts the risk range at 22,000 to 28,000 miles or 5 years, regardless of mileage. Push them further is like playing Russian Roulette with your engine valves and your safety. Preventive maintenance is the key. If you don't know when the belts were last changed, you are at risk.
On the other hand, the 1988 to 2000 6-cylinder curvilinear cog belts are not so risky and rarely break but those darn radiussed cogs just start to wear during timing fluctuates and a lot of times the bike just stops running due to sensor shut-downs when the valve timing span becomes too distorted or irregular. It's no fun sitting on the highway when you can't figure out what is NOT going on while all systems appear to start and run.
Experience with customers tells us to change your valve timing belts at a about
35,000 to 50,000 mile intervals for best performance and lowest risk of being mysteriously stranded.
So, do it! Change your timing belts! But then, we do make a bigger sale if you
gamble and lose, as lots of riders have done. Valves, head gaskets, exhaust pipe gaskets and ... new timing belts make very nice order!!
Fractions of pennies per miles, a small cost to replace and maintain as compared against damaged engines, huge expenses, towing bill and risk of life and limb.
Don't end up on the side of the road when your most critical engine component fails!