I usually plan on an hour if we are going from our resort to another resort, from one park to another or from our resort to a park. 45 minutes if we're leaving from a park to a resort. I've never missed a ressie this way- usually we were early and had a chance to look around. To get to Ohana from ASM you will need to change busses somewhere- use the parks if at all possible. For example, take a bus to MK, then catch the monorail to Ohana.