Maybe your question is difficult to answer because you haven't specified what it is being foiled. Typically foiled BBQ meats include ribs, butts, and briskets. I will, however, offer this illustrative example:
I used to be not-too-concerned about how tightly I wrapped butts when foiling them, and, on average, 7-pounders were taking 11-12 hours. I foil once they reach 150*. The last ones I did, I made very certain to use an ample amount of foil to allow me to seal them very securely. The result: 7.5 hours to reach 205*. The difference from the 12-hour version: nearly indistinguishable.