The shuttle trip can be as small as 55 minutes on weekends or as long as 2.5 hours in weekday rush hour. Unless you are in weekday rush hour 3 hours is sufficient if as the previous poster mentioned you do not get unduly delayed. I believe they raised the minimum official connection times to 4 hours now.
Go to
National Express Site where you can enter the start and end and see the entire list of routes. Now that National Express has consolidated the three or four lines that used to run between the two, there is very frequent service between the two airports.