The ideal thing to do is always have console access, of course, even if it's emulated console access through another interface.
If you're using something that uses an iptables service (like /etc/init.d/iptables) which you can turn on and off, you could set up a simple cronjob to turn it off every 30 minutes. A more general solution would be to run the following at some interval:
How to: Linux flush or remove all iptables rules.
This can be accomplished by a cronjob, or just running screen and doing a sleep loop in bash to run that little script occasionally.
If you're experimenting with iptables and learning how to use it, though, nothing beats having a monitor plugged in, just in case.