It will depend on how crowded the airport is when you arrive. Weve arrived in the evening (after 9:00 p.m) and used Mears in the past and it took about an hour. This includes (possibly) waiting in line, purchasing tickets and waiting for your transportation. If you have your vouchers, youll only have to pick up your luggage and go to the Mears location for your transportation. It will also depend on who else is in the van with you; if you have it to yourself (unlikely!) youll go right to your location. Otherwise, there may be intermediate stops. A few years back when I went to WDW on an all girls trip, there were four of us and they sent us to OKW in a Mears taxi. There was a torrential downpour going on and numerous accidents on I-4, but the driver took the backroads and got us there quickly.
Ive found that timing definitely makes a difference and that the Mears ticket counters are more crowded during the day, particularly mid-afternoon. Evening seems to be slower.