If you are familiar with the route, the driving time should be less than 15 minutes. If you are relying on the Disney road signs to get you there, I would allow time for at least one wrong/missed turn. Their sign system is not as developed as one would think it should be.
We were making that commute in reverse during our May visit and missed one of the turns we needed to make, costing us a good bit of time turning around and backtracking. I had asked on of the valets at AKL how to drive to MGM. He provided me with one of the business card sized cards that they have that have driving directions to various places. It was almost comical how little help it was. It basically said: Turn left out of parking lot, then follow the signs to MGM. Its a good thing this only set us back for a mid-afternoon arrival (we mainly wanted to see Fantasmic after spending the morning at AK). If their poor signage had caused me to be late for the early morning rope drop, I would have been ill.
In summary: Leave the gate an hour ahead of time if you havent made this drive before. If you know where you are going, 45 minutes would be plenty.
