If you've not been to any Disney parks, and also depending on the ages of everyone involved (no small kids needing afternoon naps), you can manage Orlando in 10 days, 10 looooong days, and it helps if you are staying on property to manage the loooooong days because of Extra Magic Hours.
Universal is 2 days (or 3) now with Harry Potter, though they aren't loooooong days, they're just pretty full days. Disney is at least 5 days on a first trip, one day for each park and an extra day to repeat visit at least one park. You also need one day for rest and/or shopping, and you should allow another day to go visit some of the Disney resorts. You then need another week to recover from that frantic 10 days in Orlando, but that doesn't have to be in Orlando. Or you can go slower in Orlando and take longer
If you have people that can't manage looooooong days, you need longer in Disney (Universal is much smaller, so other than Harry Potter queues, you shouldn't need longer. pre=Harry Potter Universal was 1.5 days)
There is also a lot of other stuff in and around Orlando, you could easily add another week just adding a few more of those attractions.