They say an air cooled engine should last around 5000 hours and liquid cooled closer to 10,000.
that being said you have to consider the cost of replacement as well. I just replaced my air cooled 26 HP engine for $1300 and it took a half a day to do it myself. A water cooled will be 3 times as much to replace.
I live in the desert where temperatures are 110+ all summer long. I know the difference between 80 degrees and 110 feels like a lot to us humans but an extra 30 degrees is nothing to an engine. On air cooled the oil also helps keep the engine cool and the biggest killer of air cooled machines is going too long between oil changes or letting the oil level drop too low. You need to religiously change the oil every 50 hours and check it every day. You also have to allow sufficient room around the machine for sufficient air flow. You can't have other equipment stuffed up against the machine blocking the air flow. Also keep all doors open and roll down your front windows. If you keep up on the oil and allow for good airflow the engines will run for years.
Makes perfect sense that the oil helps cool the engine also. And now that you mentioned the difference in 30 degrees between us and an engine is nothing, but I was just wondering how it would be if you had an air cooled and your client list consisted of a lot of commercial jobs where even with 2 techs it would still take a full day. And in the middle of the summer, especially in Nevada and the Southwest, and the engine pulling in hot air, how effective could air cool the engine. And yeah you're right, liquid cooled cost 3 times as much but that's mostly the components of the cooling system, radiator, pulleys, water pump, etc.
Again I'm not trying to knock anyone running an air cooled set-up, I just always wondered why liquid cooled machines cost so much and if it's really worth all that extra money and why only the big machines capable of dual wands are all liquid cooled.