[fetchmail] Demon not working?

What do you do when you want fetchmail to run for one specific user on
boot? I tried adding:

su - <user> -c fetchmail -d 60

But this dosen't even seem to download mail for that user, let alone
doing it every 60 seconds.

So I'm having to run a cron job; I just read that I could be using the
demon to "sleep <whatever> seconds between runs, so no overlapping
runs."

This kind of makes sense, as it would be annoying if it tried to
overlap.