From the few times my site (devgrow.com) has been on the front page of HN, I've gotten about 1000 visitors or so over the span of 24 hours. This has been consistent for both a post with 10 upvotes and another with 18. I would assume it would get maybe double or triple that if it had more votes and stayed on the front page longer?

/r/programming on the other hand sent over 3800 visitors during a 3-day period.

Hope that helps!