As much as you feel comfortable with, really. I think most of the mods who have been alpha-testing with me have done about half an hour to 45 min a day, or less, not counting time spent talking in the feedback area. The beta will be far less demanding as I won't be adding as many brand new features and recoding the scoring algorithm every two days. (I hope.)
The very excellent moderator team (and moderator-in-training) have been testing the heck out of it since the beginning of the year, but we're all staff members and familiar with the back end of things. I want to make sure that it works for folks who aren't as inured to the quirks of the system, and who haven't been watching me build it, break it and fix it over the past five alpha release versions.
Of course, if you get into it and find the whole thing fascinating and want to hack the heck out of it, I won't mind
What I'm primarily looking for is to make sure the scoring is fair, the instructions are clear, the features work the way they're supposed to, and that it's a useful thing. I also want to make sure that the dynamic between regular users and patrollers isn't too icky. I may have a couple of people "role play" during the beta from perspectives like a student with an axe to grind, or an employer looking to boost their score by posting bogus reviews.
We're rapidly approaching the season when students will be looking for summer internships and I want to have it up and running by the time they need it. I hope therefore that this beta will only be a couple of weeks long.
PS: We're up to 7 people! Keep it up, folks, and thanks to all who have volunteered!