On our March 2009 trip, we were scheduled to land at 2:17pm. I made a dinner reservation at CP for 5:30pm. Our flight landed at 2:30-ish. I ran ahead to get the rental car while my mom and DW managed the kids and the luggage. I had a tough time getting my DD's car seat installed in the rental, so that took about 10 minutes longer than it should have.
Because it was a trip we won, we needed to stop at SSR to get our park tickets. We drove there, everyone got out cause of the excitement and I got the tickets. They gave us our room keys, but we didn't even go there. With our luggage still packed in the trunk of the rental, we made a bee line for MK. We parked, got in the park and I ran ahead (again) and made it to CP right at 5:45pm. I begged and pleaded with the CM to seat us, and then she told me that they hold your reservation for 15 minutes after your slotted time. I told her that I could just kiss her! 5 minutes or so later, my family arrived. I do regret not being able to walk down Main St. with them and take in all the magic.
So while YMMV, it took the 5 of us 3h 45m to get off the plane, get our luggage, get the rental, drive to WDW/SSR and get tickets, drive to MK, park, get into the park and get to CP. Wayyy longer than I anticipated. I thought 2 hours max. I'll never make that mistake again.