Hi Hungry
By experience with chicken and turkey, it depends on the quantity of salt...
More salt=less time
Less salt=more time
1/2 cup salt per gallon of water for long brining (up to 12 hours)
1 cup salt per gallon for "short" brining
BTW it is no where etched in stone that you need to use water, as such...if you used apple juice or pineapple juice, you'd get some very interesting flavours, as well as the benefits of the acids which, in my opinion, do much to tenderise the meat...
Since you are using an already tender cut of loin, that would argue the "short brine" method...but remember, when you brine chops, it works through very "quickly", as they are much thinner than your roast now will be...go for 4-6 hours, in my opinion...
Feel free to add some herbs to the brine, as these can make the meal that much more pleasant an experience, and "more is better" as the herbs are pretty weak in finite quantity...
(you are heating the brine, dissolving your salts, "flowering" your herbs, then cooling, before inserting the meat, and refridgerating the lot, aren't you?)(I thought so!)
The combined use of garlic cloves and "lardoons" in slits cut in the meat (after taking out of the brine, obviously before cooking) works fantastically! Hold them in place with toothpicks, so you can dig them out before carving...
If you don't "happen" to have spare pork fat chunks to slice into "lardoons", then simply pinning strips of side bacon around your loin will work almost as well...unless you have more than normal fat on this roast...
I did this a week ago today for dinner, and wife and daughter just "inhaled" the stuff, accompanied by mashed potato's, and, of course, the gravy I was able to parlay out of the drippings...(note I was using tenderloin, but the cuts are close enough that you will likely get close results!)
You are in for a really delicious meal!
Best Regards,
Lifter