I bought my turkey at a butcher and I don't know how much it weighs.

Nancy, that is a hurdle, and I'm guessing you don't have a kitchen scale. But do you have an accurate bathroom scale? If your turkey is well wrapped and your scale is clean, I'd start there. (And if the idea of putting your turkey on there is unsettling, you can weigh yourself, then weigh yourself holding the bird, then subtract A from B.) If you don't have a scale of any kind, you will be O.K. as long as you have an oven-safe thermometer and stay vigilant about monitoring the temperature of the turkey as it cooks (15 minutes a pound at 325 degrees). The thermometer should read 165 degrees when you insert it in the thickest part of the thigh. Unless your turkey is relatively small (less than 12 pounds), you are looking at a three-hour-minimum cook time. But to be safe, you can start checking earlier.