MK you want at least 30 minutes, you don't park near the gate and need to take transportation around the lake. The rest 20 minutes is probably more than enough. This is based on the time from when your car is in a spot, not the time to get to a spot.
I also don't have little kids or a lot of stuff to slow me down, I walk a NYC pace, and I know the place blindfolded so perhaps others will chime in if my times are a little off for the average visitor.