First of all you need to figure out what your surfers need to remain a member from month to month. This is going to be different from site to site. A military stud pissing site will require significantly less updates than a 25-30yo amateur site.

But generally speaking, members who find themselves caught in a routine of logging in often and feeling satisfied with their experience the vast majority of the time are the members that retain. You should base your mission statement around that goal.

One huge factor in all of this is the amount of existing content.

There are many sites that you can look at from top to bottom in a short period of time, and then it's all stale. The problem here is that if members don't really like the last update, there is nothing for them to fall back. So they leave disappointed that day. How often is that happening to them?

If there isn't that safety net then you've really got to play it safe on the updates.