As we've said, the hardest part of maintaining your firewall is keeping yourself up to date. How long does it take to keep up to date? If you're a novice at this, just getting started and at the steepest part of the learning curve, keeping up to date can easily occupy you full time. After you've been at it for a few weeks or months, and you've learned the fundamentals of what you need to know, your time requirement can drop off to just an hour or so a day to follow the various mailing lists, newsgroups, magazines, and other sources that you've decided to track.
Most of this time will be devoted to maintaining your own knowledge, not maintaining the firewall itself. Monitoring the firewall itself should only take minutes a day: long enough to scan the daily log summaries and make sure that nothing unusual or noteworthy has happened.
Obviously, you're occasionally going to have to devote more time to the firewall when it's time to fix something, upgrade something, or add new functionality. How long this takes depends on how complex the fix, upgrade, or addition is. The better job you've done anticipating your site's needs and designing and building the firewall in the first place, the less time you're going to spend adapting your firewall to changes. Many sites find that they need to update their firewall only about once every few months. The rest of the time, it sits in the corner just humming along.