Stone,
Doug D called it right. Temp is a big factor here. I usually do spares 4-1-1, or 4-1.5-1 if the rack is a little "stiff" after the first 4 hrs.
Too long in the foil (beyond 1.5 hrs IMO) tends to make the meat too fall-off-the-bone tender, and the texture gets a bit mushy. But hey, that may be just what you're lookin' for.
When foiling, I always add a mixture of about 75% apple juice to 25% apple cider vinegar, sprayed on generously with a squirt bottle.
The last hour is for finishing with a sauce, applied lightly. If the sauce has a lot of sugar, I hold back until the last 20 minutes 'cause it'll burn quick.