Another really nifty way to achieve pretty much the same is to use iptables' built-in limit module like so:

  -A INPUT -p tcp -m tcp --dport 22 -m state --state RELATED,ESTABLISHED -j ACCEPT
  -A INPUT -p tcp -m tcp --dport 22 -m state --state NEW -m limit --limit 3/min --limit-burst 4 -j ACCEPT
  -A INPUT -p tcp -m tcp --dport 22 -j DROP

Limit-bursts allows that many packages through before any limit counter starts, if any of the other packages afterwards also do not garner any response, a counter counts up for every further package, effectively blocking everything after the first 7 packages arriving within a minute. (You could also do 3/hour, for example, if you think you need it.)

The limit module is a beautiful thing, since you can also use it to limit log messages generated for events etc., using the same syntax, like so:

  -A INPUT -m limit --limit 1/hour --limit-burst 3 -j LOG --log-prefix "iptables denied: "