In my personal opinion, it depends on a lot: a whole family vs a solo adult, how far away you are traveling from, and if this is likely going to be your only trip, even how much of a Disney fan a person is.
For a family that's likely to return again (ie, from the USA), I'd start with 5 days in the parks, with a break day in the middle (so, 6 days in the immediate area). With someone who's not from this country, I'd recommend taking advantage of the 10-day tickets that are available for overseas travelers. For what it's worth, residents of the USA are only allowed to buy up to a 5-day pass; overseas travelers are allowed a 10-day ticket. I believe Canadians have a longer ticket available as well.
If it's a single adult who really loves Disney, I'd recommend 3-4 days. Obviously, it's easier to get done what you want to get done, and enjoy it, if you are just a solo adult, so their touring tends to go a bit quicker.