This is the general recipe I use for deep frying a 15- to 18-pound thawed turkey using a propane burner as the heat source. Your mileage may vary.
I use a Bayou Classic 32-quart stainless steel pot with a standard propane burner and tank and peanut oil.
Around 45 minutes (the longer, the better) prior to frying, inject the turkey with a marinade. Tony Chachere's Creole Butter is a good choice. Some marinade leakage is expected, but don't make too many holes in the bird.
For a crispier skin, rub the bird in kosher or sea salt before frying.
To determine the correct amount shortening to use, place the bird in the pot and fill with water until it is completely submerged (preferably an inch above it). Remove the turkey and note or mark the water level on the pot. Empty the pot and thoroughly dry it and the bird.
Preheat shortening to 375°F, turn-off the burner, then slowly lower the bird into the pot. Once the bird is fully submerged, relight the burner. Monitor the temperature carefully, maintaining 350°F to 360°F for the entire cooking process.
Cook the bird for 3½ minutes per pound.
Once cooled, the oil can be returned to its original container for reuse, filtering it through cheesecloth in the process.
If you have any questions, feel free to contact me.