USDA says that size will be done in 5 hours or so at 325F. Here is a recipe for slow roasted. Calculate for your size
http://www.cdkitchen.com/recipes/recs/140/Slow_Roasted_Turkey4097.shtml
Here is a site that says it is unsafe to slow roast. I'm not sure I agree with that, if it is unstuffed. I don't think bacteria can grow and thrive at 200F. Stuffed, yes unsafe.
http://www.cdkitchen.com/recipes/recs/140/Slow_Roasted_Turkey4097.shtml

Ironically USDA says you can cook turkey at 325 from frozen and that takes about 50% longer.