Two factors that we consider are: (1) the cost of transportation to get to Florida, and (2) the fact that WDW tickets get cheaper on a per-day basis the longer you stay.
In general, we like to be able to go to MK for two full days, Epcot for two full days, and HS and AK for a full day each. We also like to be able to go to both water parks and Disney Springs.
For our first family visit in 2007, we planned a 6-night stay (Sunday to Saturday). On Friday, nobody wanted to leave the next day, so I extended our stay by another night, even though we had to switch hotels. On this visit, we were able to see everything we wanted, but the vacation was completely non-stop.
I didn't want to make the mistake of a too-short trip again, so our second visit was 8-nights (Saturday to Sunday). That worked out pretty well.
Lately we've been going on Disney cruises, with WDW at both ends. Last year we did 4 nights at WDW, followed by a 4-night
Disney cruise, followed by 4 nights at WDW. Next year it will be 3 nights at WDW, followed by a 7-night Disney cruise, followed by 3 nights at WDW.