the 50 post rule was good and might have actually helped clean up the other forums. This new policy will do nothing and the other forums are already full of spam.
I think something in between would be better: 20 posts and 4 hours logged in. It's likely that you wouldn't get to 20 posts in that time period, which would continue to deter spammers. Honestly, though, I think anyone that has a good introduction should be whitelisted: how often do spammers take the time to introduce themselves?