how long to cook an unstuffed, room temp 15 lb turkey @ 425 deg

Until the internal temp in the thickest part (leg/thigh) is at least 165; preferably 180.

At 425, it should take about 3 hours. I've been roasting turkeys at 450 for even shorter times and they are great.

That temperature seems pretty high for roasting a turkey. At 350, a 15 lb turkey should cook for 4-5 hours; at 425, it may take less than 4. Make sure the internal temperature of your turkey reaches 165 degrees for safety reasons.