Since a few years ago, it is beginning to be very common to have hundreds of connection attempts to SSH port, trying common usernames and passwords.
This has several drawbacks: log files can be filled up, SSH service can be irresponsible and, what is worst, some of the attacks could be successful if one of your users has a weak password.
To prevent those attacks, you can use these simple iptables rules that forbid establishing more than 6 connections per minute from every IP:
iptables -N SSH_CHECK
iptables -A INPUT -p tcp --dport 22 -m state --state NEW -j SSH_CHECK
iptables -A SSH_CHECK -m recent --set --name SSH
iptables -A SSH_CHECK -m recent --update --seconds 60 --hitcount 6 --name SSH -j DROP