The American Dental Association and most dentists recommend having bitewing radiographs taken at least once every 12 months. That is because the bacteria that causes cavities, if left in between your teeth (from either lack of flossing or poor technique) can pit your enamel in just a few weeks and in 2-3 months they can eat through the enamel and get into the dentin of the tooth.
Below you will notice 2 bitewing radiographs. They are the same radiograph. In the first one you see, I have highlighted the interproximal decay to help train your eye on the area with the cavity. The second radiograph is the unaltered bitewing. For decay to show up in a radiograph it has to lower the density of the tooth structure (by dissolving the tooth with acids produced by the bacteria) 40-50%.