Sixty to ninety minutes, but expecting to squeeze in ANY park activity (unless there's an extra-late EMH) is probably being overly-optimistic.
First, this assumes your plane lands on time. A recent report indicates that as any day goes on, flights are more likely to be late.
Now, that 60-90 minutes is from getting off the plane to getting off the bus at your resort. This means ideally you'd get to the hotel between 8 and 8:30 - figure 8:15. You need to check in; how long this takes depends on how many other Guests are checking in and how many CMs are working. Even if you go directly to the bus stop, you'd still need to check any carry-ons with Luggage Assistance. Park buses come every fifteen or twenty minutes. It takes about twenty minutes to get to MK.
