How often do you do informal retrospectives?

Recently I've been doing informal retrospectives at the end of each 'development' session. (Just keeping the notes in Google docs for now)

Usually it's just writing down what I've done, what I'd like to do next and the things I hacked and need to fix. (or reminders to do something that I've been meaning to do for a while)

Does any one do anything similar? Or maybe with a different format?