Michael Lopez, Web Design Lead at Google, answered a few questions about Google's doodles. He says that it usually takes 1-2 days to design a doodle and that Google's homepage is "the biggest little canvas an artist can display their artwork on".
The doodle selection process aims to celebrate interesting events and anniversaries around the world that reflect Google's personality and love of innovation. We know that our list of doodles is not exhaustive, but we try to select doodles that show creativity and innovation in a fun, quirky way. Generally, we choose doodles from a variety of categories, such as those that celebrate holidays, anniversaries, and the lives of artists and inventors.
To see a gallery of Google doodles, check this page. Many of the doodles are only displayed in certain countries, so you should subscribe to Google's feed if you don't want to miss some doodles.