Do you remove peel and wrap it in tin foil ?
Do you put it directly on gril (with peel) ?
How much time ?
The best way I have found is to soak the ears in husk for several hours before grilling. This lets the husk soak lots of water. Then place the corn, still in husk, on a hot grill for about 10 minutes, ~1/4 turn, 10 minutes, turn... until the husk gets brown, even burnt.
You should be able to tell when the corn is cooked by the smell. The sugars in the corn will start to caramelize and brown in the husk and it will give off a delicious caramel smell. You can check them by peeling down the husk to make sure that they are cooking well.
Times will depend on your grill and how much patience you have. In general, it is difficult to overcook. You want to watch out for drying, but that usually takes a while if you soak the ears beforehand.
Foil is a poor substitute for the natural wrapper. The husk both protects the cob from too much direct heat as well as holding water close to the cob to gently steam the kernels. And its just prettier with the browned and burnt husk, au natural.
I've done it a few different ways --
husked, desilked, over indirect or low heat : my current favorite method; brings out the sweetness of the corn without the grassy qualities; have enough time to turn it without charring, but if you're late turning, won't instantly turn into charcoal. Takes maybe 15-20 min. Still works for corn that's been sitting for a couple of days, without much prep.
husked, desilked, straight over direct heat : had to work quick, or they'd get overly charred, but very easy to prep, and cooks really quickly.
husked, desilked, wrapped in foil : not bad, less likely to char, but I'm lazy and it's extra work (and quite a bit of foil if you're doing a dozen ears).
husk on, but trimmed the husk and silk that went past the ear : (unsoaked; the ears were fresh from a road-side stand) : No bad, still had to de-silk after cooking, but gave it a grassy note which wasn't my favorite. (if you're a fan of green peppers, you might like it).
husk on, opened, de-silked, then wrapped back up in the husk : much more effort than other methods; gives the corn a grassy quality to it
Now, if you're cooking over campfire ... then yes, I'd soak the husks, or go with aluminum foil ... but a grill where I have more control: husked, straight on the grill.
In the past, I've soaked it as per an earlier suggestion, but not for several hours since my grilling is usually relatively impromptu. I soak it for as long as I can get away with (usually at least half an hour), leave the husk on, and drip a little bit of melted butter on top of a foil sheet, then add some simple seasoning (garlic powder, salt, pepper, whatever) on top of the butter, then wrap it up and grill it for about half an hour, turning every 10 minutes or so. I'm not sure if the butter and seasoning helps, but it doesn't hurt, and you're not losing much. It's certainly not the BEST method, but it works for me.