What type of meat are you thinking of brining?
Well now, seeing that you posted this in poultry...disregard my question above.
Like Al said, 3-4 hours.
If possible, overnight for whole birds. Don't go over 24 hours or else the meat can get mushy.
Bone in, boneless, skinless, pounds, etc... it really doesn't matter. Keep it covered with the brine and chilled in the fridge the whole time.
Rinse and rinse again when done brining.