Usually the default values are fine. If I'm not mistaken it is somewhere along the lines of 10 in 5 seconds.
I don't use BFD on my production box however because you are always going to have that customer that leaves "anonymous" in their FTP client and can't figure it out. Then their "site goes down" because their IP has been blacklisted and can't send a support ticket because you run the helpdesk software on the same box.
That, is a very pissed and possibly no longer a customer type of customer