I always budget (because I am an eternal pessimist with travel) approximately two hours from wheels down to leaving the DME bus at my resort. It is generally shorter and averages about 80 - 100 minutes from wheels down to departure but, if I am planning anything (ADR or otherwise) in advance, I assume we will not get to the resort until 2 hours after landing.
It is just no fun to stress and I don't like to cut things short. If I plan on lots of time, I am rarely disappointed with the time it might take. We remind ourselves we are saving over $100 by using DME vs a towncar or rental.
As for the wait time after check in, it has varied from ready at 10 AM to not being available until 4 PM. Again, plan on it NOT being available until 3 or 4 and then be pleasantly surprised if it is ready.
You can always store your luggage with a bellperson and go enjoy a park, eat, shop, swim, etc. before your room is ready.
Have a great trip and if you have any other questions, please feel free to ask.