On farm and forestry use we regularly see Hondas with 25,000 miles and more, and with thousands of hours on them. We reckon that with regular oil changes that the head should never come off before that mileage, and very often much more. These are working machines, doing often short runs - but could be towing, spreading fertilizer, spraying etc.. The oil changes are what makes the big difference , plus - don't drive them hard from cold, let them warm easy (except when brand new-but that's another topic for another day)