The bus ride itself is about 30 minutes. If I make a reservation on arrival day I always budget at least two hours between plane arrival time and reservation time, depending on the location of the reservation. This time we arrive at 2:30, the rest of our party at 4:40. I'm making a reservation for Sanaa (at our resort) for 6:45 or 7 pm so we can all eat together. Factor in time to walk from the gate to check in for DME, stand in line and wait for the DME bus, drive to about 3-4 resorts (depending on where your resort is in the drop off queue). We've had the whole drive take more than an hour when we were the last stop.
Edited to add, if your ADR is at a park, budget in travel time to the park too.