If you are looking for a time period as a guideline. I believe it is 10 years.
No hard facts but I feel a well used unit will be showing its age at that point.
It will be easier to trade the unit vs, refurbishing it.
I do believe with proper care and preventative maintenance an RV can easily last more than 10 years,but 10 is a good guideline.
As Dutchmansport mentioned above there are lots of reason to trade that are not based on time or years of ownership.
While time of ownership is often a factor it is seldom the sole deciding factor in determining the trading point.