500 hours of video per minute = 30000 per hour. If a moderator watches 30 hours per week that'd be about 0.2 hours per hour. So you'd need 150,000 moderators to watch every minute. Given that billions of people watch youtube, this is one moderator per ten thousand or more viewers. Also considering most of those videos are not going to be watched by more than a handful of people, you could probably get away with a tenth that by only moderating videos with say 100+ views.

So about 10,000 moderators required. This costs under a billion per year. Given that Youtube brings in $20B + of revenue, I'm sure this would not kill their profits.