Do NOT cook on a set temp. Repeat NOT.
If you're new to smoking, new to cooking or new to doing something you haven't done before. Time will never serve you well.
The only method is to get a remote probe and go for a set internal temp.
You friend might know his smoker his methods, etc, but just the fact he told you that tells me he doesn't know enough to be more worried about the finishing temp.
Read Turkey 101.
Oh, and the time/duration depends on the temp of the smoker. As well as the size of the meat. And how many times you open the door (and let heat out).
My motto, many others have stolen, but it will serve you well to remember:
It's done when it's done Just means that it has a mind of it's own and you need to learn how to read it's mind.
You'll get there. Just practice, take good notes and don't listen to your neighbor