It kind of depends, your time could be longer depending on when you are arriving and the time of year. If you are arriving first thing in the morning during a busy season, the line to get on the monrail takes longer. I normally plan on 30 minutes at least.
If you are arriving in the morning and it is morningEMH for on-site guests, you will have to wait until 10 minutes prior to park opening to get through the gate for the monorail. A very large crowd builds, so add in extra time for that.