I have a cinderblock pit and have never had a pig take over 12 hours.
A lot of it depends on the weather and the size of the hog.
I place hot coals (whether they are charcoal or coals you burn down yourself) under the hams and shoulders on each end of the pit. The middle or rib section of the pig needs no direct heat.
I add a few hot coals about every hour...most folks check the pig and think it's not cooking fast enough so they add too many hot coals.
The idea is to cook it low and slow...just add a few hot coals at a time to avoid a flair up.
Like Norco said, you can marinade overnight.
When I marinade a pig overnight, I use a large plastic watering trough, the pig lays down in the marinade in the trough and I cover it with bags of ice.
Sometimes I marinade in a home made mojo, also with lots of garlic cloves embedded into the meat.
While the pig is cooking, I like to use a mop of vinegar, onions, red pepper flakes and jalapenos.
I agree with skidder....a covered pit will cook faster. They are easy to do and very tasty.