What proportion of your income should you spend on rent?

I’ve heard a lot of different angles on this one in the last few days, so now I’m opening it up to the floor. I once heard someone say that you should pay one-third of your income in rent. That’s all I can remember. One third on rent, one third on something else and a final third on something else again. Now of course my first question is whether or not that’s supposed to be pre-tax salary or post-tax salary. And today a friend in the US said that you should never pay more in rent than one week’s salary. Thus leaving three-quarters for living and saving. So what’s true? What’s the correct ration to spend on a place to live? Won’t somebody tell me!?