JohnMu from Google posted in a Google Webmaster Help thread that Google typically crawls a site’s robots.txt file on a daily basis. This is the first time (at least that I can remember) I have seen a Googler make a statement on the crawl frequency of robots.txt files.

JohnMu said:

I have not validated this with my sites log files, but that is not the point. The point is a Googler said, on a general level, how often Googlebot will refetch a site’s robots.txt file.

Forum discussion at Google Webmaster Help.