This question seems to be asked many times by people who are starting a blog. John Chow recently wrote a post about this and according to him, this is one of the most asked questions he receives. Darren Rowse from Problogger also wrote about this, after a number of readers had asked him this question. Both posts are very informative and are definitely worth reading.

My personal preference is to put ads on a blog from day 1 and that’s what I would recommend. However, you need to be smart about it. At first, start with only one ad and gradually add more later as your blog gets more readers. The amount of ads should be proportional to the amount of traffic a blog gets. It’s fine to put a lot of ads in a blog that gets an insane amount of traffic, but once again, be smart about it. Ad placement is very important, but I’ll save this topic for another day.