DrewE's post made me think about some of my early days camping, and Vermont State parks.
Many state parks, and this is especially true in Vermont, were established many years ago when camping was done with tents. The sites were sized to accommodate tents and were a lot smaller than sites in more modern campgrounds. A lot of these state parks still use the same sites, that's the way the trees have grown over the years. The same is true in many other areas.
So, how long is too long? That depends on a lot of things as many other responders have already indicated.