We went last September with our son who was 17 months at the time, and are scheduled to go again this September now that he is 29 months old. Both trips we are doing are 8 days/7 nights. First, you need to understand the temperment of both your toddler and you as parents. How are they around people, are they good in restaurants, do they handle these type of settings ok. That said, do not push them -- keeping their routine is important so try to honor their nap times, some quiet time, pool time, etc. You cannot do WDW the same as you did before kids. Make sure you find some down time as parents as well. My wife picks up little toys -- from Disney store, dollar store, Happy Meals, etc. -- packs them in individual zip lock bags to take on trip. We make sure we have different distractions for my son at key times like on the plane, during meals, while watching fireworks or whatever.
I do not think there is a magic number of days as long as you take it easy and try to keep toddler on schedule. If you want to see it all and are trying to do it in only a few days -- you and your toddler will go over the edge. I also don't recommend the philosophy of "they will sleep in the stroller"? Kids need some good rest.
Finally, be flexible with your schedule. Toddlers moods change quickly -- if they don't want to do something, move on. If they are not cooperating in line or at dinner, leave and be considerate of those around you. One other recommendation -- you never know how they will react to the characters. One day they will love them, the next they will want nothing to do with them. Do not push them -- and don't get upset if they only want to look at the characters from a far.
We had a great time last year -- my son still looks at the Disney scrap book of our trip -- we had a blast. As such, we are going again in a short 32 days!! Have fun.