It's possible but not very easy. You have to be constantly posting content that gets people to click on ads, which isn't easy at all and keeps changing all the time. There are lots of content sites that let you write for other people though and get paid up front, and also lots of revenue sharing sites, so if you keep at it, you'll get there.
I know people who make as much as $1000 to $1200 per month on Adsense, but if you think about it, that's only $12,000 to $14,000 per year before taxes, and you work all the time.
A better approach is to a bunch of different things--some that get you money up front, some that take longer.