The default sshd configuration might be to listen only on the loopback interface (lo). If that's the case, you will need to change it. That's probably the easiest thing to look at, but if everything else were OK and the problem was just that there was nothing listening on that interface, the response should have been a TCP RESET and not a timeout.

"Connection timed out" means there was no response at all to the connection request. The packet was simply dropped somewhere along the line, and the only likely candidates are your router or the firewall in your computer. To help pin that down, you'll need to install the wireshark network analyzer. You'll need to start wireshark with root privileges ("sudo wireshark") and click through the warnings (Debian doesn't like having you run it as root). In the "Filter" box, enter "tcp.port==22" and click "Apply". Then select your network interface and click on "Start". Try the connection from CanYouSeeMe.org again and see if anything shows up. If your router is forwarding the port properly you will see several SYN packets, and that means the problem is either in your firewall or the sshd daemon configuration.