Hi Pascal,
How long one keeps an RV seems to depend on a lot of things. I suppose whether you can afford to change is a high first priority. Like a car, a new coach depreciates as soon as you drive it off the lot. If you can afford to upgrade early you might do it, but you won't if you can't afford it.
Second is probably how well you like your first RV. I've met folks who see one they think they like, buy it, and trade it in within a few months because it doesn't suit their needs. In fact we met one couple who attended an RV show, fell in love with one top-of-the-line motorhome, a couple of months later went to a factory-sponsored rally, fell in love with another model of the same line, and bought it. At the very next rally a few months later the same thing happened, so they bought three top-of-the line motorhomes in less than one year. Is this normal? Definitely not! Maybe they had more money than common sense because they took huge losses on every trade-in. On the other hand, when we bought our first one way back in 1972 we didn't have much money to spare so after attending numerous RV shows bought one at the low end (also called entry level) that we BOTH really liked and was of decent quality. We had that one for maybe four or five years. We then upgraded and had that one quite a while, upgraded twice again, and after a few years got our current coach. This is five motorhomes over a period of 35 years that included quite a few years when we were still working. In fact, the first four were all bought while we were still working. With each one we had a better "feel" for what we like and dislike. With the last one we had a lot of changes made by the manufacturer (some make changes and some don't) and now have the features that are most important to each of us and what we both really like. With every RV (whether motorhome or trailer) you have to make tradeoffs and compromises, but if you take your time you probably will want to keep what you have for a while because you like your choice. That said, the first RV is probably not the last because you can't possibly know with the first one what will work best for you and your family.
A third issue is that what suits you while you're still working may not work after you retire and have more time to travel for extended periods. Many of our Forumites initially bought at the low end and eventually moved up to a higher end coach because they decided to full-time or do extended-time travel after retiring. The coach they had when they retired didn't suit their new lifestyle so they upgraded to one that was more suitable for living in every day. So part of the answer lies in what phase of life you're in currently. If you have a young family with children you probably won't want what a retiree wants because our needs usually are different. An example is how many an RV can sleep. Retirees may travel as a couple whereas a family of five needs more beds. Another example is that full- or extended-timers need to have drawers for their paperwork because they still have to pay taxes, bills, and the like. People who are working and taking short vacations don't need to bother with all that paperwork while traveling so can use the space in a way that suits their needs better.
I'm sure others will be adding to this, but I think most of the Forumites will agree they don't change RVs very frequently.
ArdraF