I would think there are far too many variables involved. Quality of build, state of tune, how hard it's used/abused, maintenance, etc. Also, what would be considered a rebuild? Some bikes are particularly hard on valve guides, for instance, and may require head work in 20-30,000 miles. Others may simply need to have the head pulled to remove carbon buildup. I would hazard a guess that the most important factor is good quality clean oil. The only one I've had a lot of miles on was a '74 Bonneville that I bought with around 40k miles on it. It was obviously quite neglected. I put in pistons and rings, had the cylinders bored, the head reworked, and rode it about another 30,000 before the engine had to come apart for a spun main crank bearing. One of the members when I was in the North Texas Norton Owners club had a Bonneville that went 90,000 on synthetic oil (he was an aircraft mechanic). When he tore it down, thinking that it HAD to be worn out by then, he claimed all was well in the lower end.
