Blogging for Money is not as easy as the hype that goes around. It is certainly not a matter of simply starting a blog, putting some ads on it and waiting for the monthly checks to arrive at your post box.
This recent post on the Alexa Blog presents an estimated calculation on possible earnings from blog monetization, serving as a reality check on money blogging. It is estimated that, despite the economic setbacks, the online ad spending in US alone will expand to an impressive $24.5 billion in 2009. The figure worldwide will be much more and a portion of that money can be grabbed by all the bloggers out there. The problem, however, is that it needs lot of hard work from the part of the bloggers, to be of any success. The above mentioned calculation is somewhat narrowly scoped in that it only considers CPM (Cost per Thousand Impressions) ads, but the overall message it conveys is still true.
So remember, if you want to monetize your blog, don't expect anything soon. Be prepared to invest lot of time and effort to build up your visitors by offering quality content and being responsive to their questions etc. It may require a year or two for you to be successful. (The time period will, of course, vary depending on your blogging niche and the amount of effort put)