I left my mustache growing for a long time. When I was a sophomore in Junior High, I heard the first people say: "Ed, you have to shave." Yet I didn't shave until six years after that. The reaction of the people was quite... surprising when they saw me with a shaved face for a change.
Nowadays, I shave twice a week. Shaving once a week is fine too, but it gets a bit too noticeable during the end of the week. I tend to forget it too, so...
I don't have a fast beard-grow though. I could easily not shave for a few weeks without someone making a too big comment on it. My brother on the other hand, he is the complete opposite of me. In truth, he could shave twice a day because it gets very noticable. Unfortunately (for him), his skin is too sensitive for shaving it more than once a day.