ost of this time will be devoted to maintaining your own knowledge, not maintaining the firewall itself. Monitoring the firewall itself should take only 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.