Not an insane amount of money, of course, but enough that you can consider the project successful if you're getting a solid amount of views.
My largest project, http://sleepyti.me, gets about 1.5 million unique views per month. The revenue Google Adsense brings in is not nearly enough support myself, but it's enough to make the effort feel solidly "worth it" in terms of development time and hosting costs (which are very low at this point).
How (or if) you should be monetizing depends on the nature of your side project. If your "side project" is a business -- say, designing WordPress themes -- then you should sell your product! If it's something that gets 50 views per month, maybe it's not the best candidate for monetization (and is instead a portfolio/resume builder). Either way, gaining experience building things is almost always a good thing.