That is not correct, Matthew. That tells us what your watch gradually does over a period of time - say a day.
'My watch is seven minutes fast' means that it is showing a time which is seven minutes ahead of the actual time.
Freeguy, I don't know how many different ways there are to say this, but they will all be wordier and less elegant than this just about perfect sentence.
Not a teacher nor a native speaker.
As far as I know (and heard) we normally say "My watch is five minutes ahead/behind". Correct me if I'm wrong, please.