It can vary for each size jig you are trying to use but here is a very accurate setup that we use to great success...
Fish with 6 pound smoke fire line and tie two 1/8 oz jigs (we use Kalin's 2" grubs mostly on them) about 20 inches apart. With 40' of line out and trolling at 1 mph (gps) your baits will be at 12 to 14 feet depending on current and other minor variances in speed etc. 50' of line and your at 16'. This formula DOES NOT work with mono line as the friction from the mono will bring the bait more shallow and the mono has more bouyancy. The same speed and setup with mono and you may need 100' of line out to reach 12 feet. Heavy mono and you will not be able to get the lure down unless you slow way down in which case you will also not get as many bites.