I am not going to tell you this is a good idea. So, if that is where you are hoping to get, you'll have to get that opinion from someone else.
But to answer your question I don't think there is A Number. There is Too Long, and Not Too Long. And even if your personal window is Not Too Long, that doesn't solve the each-point-has-only-one-trip problem.
But, I think your window is Too Short in order to have perfect plans (which you need for every-three-years), and by a lot. Here's how I think about it: When I first bought (non-
DVC) timeshares, my kids were in early grade school. They attended Ann Arbor Public Schools, which is the school district in which my employer (U. Michigan) is located. At the time, they had a mid-winter break that lined up with Michigan's spring break. We had taken a couple of Disney trips, and expected we would go to Florida every year for that week until they left home.
That lasted all of two more years before AAPS changed the week they took off to be the week
before Michigan's. I had an excahnge into a 2BR at BCV for that year that I had to cancel, and I am still a little resentful about that. For the next several years, my wife took the kids down to Florida for a theme park and/or
DCL vacation for AAPS' break. I would fly down about a week later, we'd spend the weekend together, they would fly home, and I would stay for my week.
Needless to say, that
completely changed the vacations we all took.
That was far from the only change that impacted our plans. In years I was in administration or on sabbatical rather than in the classroom, my travel schedule was more flexible---but that meant traveling at higher-cost times. The kids' schedules changed unpredictably as they got older and started to have interests of their own: sports, the arts, etc. which reduced the trips dramatically. My then-wife and I separated for three years, we tried to reconcile for another three, and finally got divorced.
I would be hard pressed to find a time when plans I made for three years in the future were reliably what I did, and that is true even after the kids left the house.