This year I've decided to make Turkey breasts instead of the full Turkey. My idea was to flatten out the breasts and put turkey stuffing in the middle and roll the meat around it. My question is, will I need extra time for the stuffing? Or will just cooking for the turkey breasts be enough?
If you are stuffing only a turkey breast then it is best to weight the breast after stuffing and calculate the cooking time following the normal formula for chicken (20 mins per lb/45 mis per kg plus 20 minutes) based on the stuffed weight. Even if your stuffing is not a raw meat type, it will suffer from cross-contamination when it is enclosed in the turkey and will need to be cooked to a safe temperature. If in doubt then use a digital (instant read) thermometer for checking the internal temperature of the filled breast, it should be 76c/165F though if you are resting the breast after cooking then you can take it out of the oven around 2c/4F below this and the internal temperature should continue to rise slightly as it rests.
For whole turkeys it is generally not recommended to stuff the cavity of the turkey nowadays, as the extra cooking time required for the stuffing can cause the rest of the turkey to become overcooked and dry. Instead put the stuffing in dish and cook it separately. However you can put a little stuffing in the neck space of the turkey (make sure it reaches the correct temperature before serving) or a thin layer between the breast meat and skin, as in Nigella's Sausage Meat-Bosomed Turkey in Chrsitmas (p115).