I wash mine about once a week. If it gets any longer than that it is a bit feral. My hair doesn't go oily... just looks a dirty colour (I have blonde hair).
My hair is pretty fine, and until recently was halfway down my back. Just cut it to below my shoulders, so might have to wash it a bit more often I don't straighten it, unless I'm going somewhere 'special' so it only gets a blowdry