If you are staying on property...my guess is about 4:00 to 4:30...breaks down like this...
1:45...assuming your plane arrives on time it will probably take you a half hour to 45 mins to get your bags...2:15...carry your bags out to the curb and catch the shuttle to your rental place and get the car...45 mins to an hour...3:00...drive to WDW, park and check in about an hour and a half...4:30...hit the parks...just an estimate...I have never rented a car...always took shuttles.