AFAIK, ipfw doesn't have dynamic rules. Alternatives: some sort of an add-on filter (and I haven't tried that one on OS X!) or fail2ban. Move the ssh port somewhere weird, an approach which will confound many of the gremlins. Maybe port knocking. Or move to a VPN. Delaying the ssh connections won't help against a distributed ssh brute-force attack, either. There's a reasonable general (non-OS X) write-up here. Personally, I find a gateway-based VPN server is usually easier to deal with, and it keeps the OS X logs from getting filled with noise.
ipfw has been deprecated in OS X in favour of pf, which does support dynamic rules and throttling. Unfortuantely, I don't work with it, so I can't actually help you with the syntax, but I do know that what you want should be possible.
However, I do also feel obliged to point out that the majority of "best practice" guides I'm aware of would all suggest using a tool like fail2ban or DenyHosts, as MrHoffman has already mentioned.