You really just need to pick the resort and then look at how much the most expensive 2BR is. I know I personally won't visit WDW during Spring Break/Christmas week which are the most expensive times to travel so I didn't accommodate for that. I know I like RIV personally and the highest the
point chart goes for a week that's not in those 2 time periods is 410 points so ultimately my goal is to get my RIV point total to 410. This guarantees I can afford a 2BR for a week at any other time point during that year. Points charts are subject to change however so it's also worthwhile to add a little bit of a buffer to accommodate so maybe for me 430-450 RIV points might be what I need. Anyways, I advise you to do your own research using either the point charts or a point calculator. Another thing you could do if you know you're not going to visit every year is to take an average of all of the travel periods it costs you and then base your decision on that if you don't want to have extra points.