You will get as many different answers as there are different people.
We are beginning our 15th year fulltiming this month. We started with a 31' 5'er that we kept for nearly 10 years. We traded (at its age 16) for a shorter 28' 5'er so as to be better able to enjoy the smaller, out of the way sites that we prefer. We'll likely keep it until it falls apart or we do. No plans to ever stop traveling. We plan to die out here on the road, hopefully not literally.