I think it should be around 3-4 months of time rather than a full year. 3 months makes it pretty obvious that they didn't join the site just to sell something, especially if they still have 200+ posts.

I'm not sure how I feel about the required rep points, as these are more often than not given out for telling a joke, they don't really represent the character of the person at all. If anything, the more rep points you have either means:
A) You're a big name on the site

B) You tell jokes and "own" people (even those who have done nothing wrong) and other people find it funny

C) You start threads about things that people really like (videos, pictures, porn)

But generally it's B

I think if you required them to be higher than 10-20 that would be plenty.

But those are just some suggestions.