Unfortunately for me, it has been very stable... And I am trying so hard to improve too... I guess if I don't ... it might drop...
I beleive what Jack is saying is that it should be on a upward trend. Of course there are going to be days lower than the day before. But the trend should be upward the longer your site is on the internet. If you niche takes a dump tho.........
It depends on the site' type. If you have a user generated content site, a images hosting, text generators... or kind of (not a content based site) ), you haven't to spend much time in your site.
If you are asking what happens to a site if you stop updating it, then the answer is that it will continue to make roughly the same amount for a while with no input from you. It won't just die overnight. If it is topical, after a while the content could get out of date, and the income drop. But if you built up a steady $1000 per month income, then left it all alone for a few months, I bet it would still be earning between $800 and $1200 at the end of that time. As Jack says, income tends to increase as more and more people make links to your site. But it can go down too if other sites start out-ranking you.