What I am always striving for with any of my blogs is to reach a level where they can hold their own for weeks on end, My Paranormal Life is my only blog at this level, I can leave it for weeks and be assured that my numbers wont drop.
With most of my other blogs, if I am not actively promoting and updating them, traffic will fall to pretty much nothing.
I put this down to a few things, the niche isn't specific enough, the niche is over saturated, they are fairly new and I haven't promoted them as aggressively as I have done with MPL.
As time goes by with these blogs I will see a few things happen, my content will grow, I should get more and more back links, my PR will grow and referrals from Google will make up a lot of my traffic.
It isn't very hard to get to the level MPL is at, 200-300 visits a day, making $2-$3 a day.