I've run my china "happy time" motorized bike for 7-8 hours non-stop in very unforgiving conditions-mainly on forestry service/access roads and atv trails in the mountain. When I stop to tighten the front chain every 2-3 hours on my SBP shifter kit (really guys at SBP, you should fix this issue with a chain tensioner) I leave the bike running. I have two gas tanks and carry about 1.5 gallons of fuel and another 1/2 gallon in my backpack.
Yeah, those china HT engines are junk
Did I mention, until someone else steps up, I'm the unofficial high altitude record holder on a HT engine. Took mine over Cottonwood pass twice last year 12,126 feet AGL on top and a very long and refreshing ride and my china HT had no problem on the 10% grades! I did have to stop at 10,000 ft AGL and move the needle clip one notch to lean out the mixture.
If you build it right, a HT will last thousand of miles. The only problems I have had were with my SPB shifter kit-front chain loosening and sloppy freewheel crank. My DAX engine is almost 5 years old-purchased in 2008- and runs like a swiss watch. My other engine is at least 3 years old and is an Ebay special.
Oh yeah, I ride mostly full throttle up steep mountain grades at slow speeds.....that HT engine can get quite warm in these conditions.
In 10 year of buying, building, and occasionally selling motorized bikes, I've had only two engine failures:
About 7 years ago, I lost a piston/head assembly when the small pin in the piston that prevents the rings from moving fell out and allowed the ring to rotate and catch on an exhaust port (I now epoxy this pin in the piston if the piston has one-some HT engines don't)
Second failure was wrist pin cage bearing failure about 4 years ago. My fault for not replacing it but it was a quick build.
If you buy a HT and slap it onto your bike as is, then you will probably experience problems.
The secret to engine longevity: 32:1 with 1/4 to 1/3 of your oil being pure degummed CASTOR oil. Nothing beats it for engine protection and though it burns a little dirtier, I've never had an issue with it as our engines don't have power valves in the exhaust.