Way too many variables. If you knew how warm the chicken was before cooking, how hot the cooking area is (and whether that will change during the cooking time), how thick the chicken is, whether you're cooking white or dark meat, and more, you might be able to make a good guess.
I would not recommend cooking just thawed, thick pieces of chicken on an uncovered grill. Too easy to make it hard on the outside and dry on the inside.
I wouldn't estimate lower than 20 minutes, but it will almost certainly be longer.
Edit: Here's a "Good Eats" video about doing chicken on a grill. http://www.youtube.com/watch?v=aAvy4YksJgM&feature=related It's better to go ahead and cut the chicken up the rest of the way for grilling so that each type and size of meat can be on it's own cooking schedule.
If you let it sit in the sauce overnight then it's no longer a sauce, it's a marinade.
That's fine, but marinating is something you generally do with tough, cheap, and/or dry cuts of meat such as chicken breasts or top rounds (beef). For a full chicken, especially the wings, drumsticks and thighs, a marinade is entirely unnecessary and in my humble opinion simply dilutes the deliciously moist interior and often makes the whole thing soggy and depressing.
Generally the goal (or at least my goal and that of the majority of restaurants I've been to) for chicken wings or even a traditional roasted whole chicken is a nice crispy, golden-brown, well-seasoned skin with juicy meat on the inside.
You can't get that if you douse it in sauce before grilling it. The water in the sauce will inhibit the Maillard reaction that causes browning (and crisping), and as Mike points out, the sauce may even burn. In fact, it almost certainly will burn; most store-bought BBQ sauces only last a few minutes exposed to direct heat.
With that in mind, the best time to sauce a grilled (BBQ) chicken is after it's cooked, or more precisely, when it's nearly cooked. Go ahead and season it liberally and coat it in butter or oil beforehand, but wait until it is no more than a few minutes away from being done before you sauce it. You're not trying to cook the sauce, and it takes no more than a few minutes to get the sauce to bind to the skin - i.e. a few coats with 30-60 seconds each to reduce and form a sort of glaze.
This is true for almost any meat and any cooking method as long as you're not marinating - BBQ chicken, fried or convection-baked wings, smoked or oven-braised ribs - you almost always want to sauce it right at the end. If your technique and ingredients are good then the meat, not the sauce, should be your main attraction.
Best Answer
That's it.
If it's properly sliced, it'll be tender, as there isn't enough internal structure left for it to resist it falling apart.
Even though it's not technically the correct way to to cook fajitas, I'd go with a mix of Bruce's #1 and 2 : simmer it 'til warmed through in an appropriately flavored liquid (a mix of lime juice and any generic italian salad dressing works well), then drain, and give it some some fast heat to sear.
Of course, this assumes that they're already cooked ... if they're not, then just marinate, cook, and slice (again, against the grain).