If there is one statement that you will read here over and over again it is:
Use a thermometer to determine the doneness of your meat.
There are many "formulas" depending on bird weight, cooking method and - frankly - gut feeling of many cooks, but for a formula to reliably work you would have to factor not only the weight of the bird, but also starting temperature of the bird, build (lean? chubby?) of the bird, temperature of your oven or frying oil, the heating ability of your oven or fryer or, beware, the coals you are roasting the bird over and almost everything else except the lunar phase.
Is this possible? Sure, and I guess some weird physicist1 has developed and possibly even posted it somewhere. Do you want to do the calculation when you are preparing side dishes or isn't it easier to poke a thermometer in the bird's breast and thigh and be done with it?
The formulas serve to give you an estimate of how much time you should need for a bird - give and take a bit.
So to get a ballpark number according to Paula Deen:
Three minutes per pound plus five minutes per bird.
Start meassuring a bit before that and remove your turkey when it has reached 150-155 F, carry-over cooking during the subsequent thirty minutes resting time should let the bird comfortably reach the target temperature of 165 F.
1 No offense to hard-working physicists - I picked just the first cliché that seemed to fit.