Specifically for the SpaceX SES-8 mission going on right now. But, generally, how long does it take for a satellite to reach GEO at a minimum?
In theory, the shortest practical time from spacecraft separation would be about five and a quarter hours, which is half of a geosynchronous transfer orbit. At apogee, which is carefully placed to occur over the equator, a single burn raises the perigee and changes the inclination of the orbit, and you're there. All early GEO birds used this approach, and many launches still do, especially from near the equator (Ariane, Sea Launch).
SES-8 went to a supersynchronous transfer orbit, which allows for much lower $\Delta V$ to change the inclination, since the spacecraft is going so much slower at apogee. It would be 1800 m/s from GTO, but it's 1500 m/s from SSTO. That includes some inclination assistance from the Falcon upper stage, which reduced the inclination from 28° in the parking orbit to 20.75° in SSTO. The SSTO in this case is 295 km x 80000 km.
The SSTO approach takes longer, and there are several maneuvers required. But it is usually well worth it to save that fuel for extending the useful life of the spacecraft. Lifetime is money. A supersynchronous transfer orbit was first used in 1992. SES-8 will perform five maneuvers over two weeks to get into GEO.
Less than 30 minutes
Falcon 9’s second stage single Merlin vacuum engine ignited at 185 seconds after launch to begin a five minute, 20 second burn to deliver SES-8 into a temporary parking orbit. Eighteen minutes after injecting SES-8 into that orbit, the second stage engine reignited for just over a minute to carry the satellite to its final GTO.