I wouldn't go much above 12,000 miles for any oil, whatever the fuel. At that kind of mileage the oil will be degrading because of the mechanical 'chopping' effect on the long chain oil molecules. There's a difference between what's best for an engine, and what's best for sales when you're selling to fleet managers, who will be typically selling the vehicles on after two years.
What you won't have is anything like the amount of contaminants in the oil, since you don't have LPG condensing out of the mixture in a cold engine and contaminating the oil, unlike petrol, or anything like the amount of acids formed in the combustion process that end up in the oil.
It can be seen when you change the oil. That from an LPG engine often retains some of the golden colour of what you put in.