I was told 25%. It's definitely true, though my housemates were certainly unable to meet that target at the salary range available to them. For reference, our combined rent was somewhere between $2100-2250, I forget exactly (4B/1Ba for those playing along at home)

If you're over 30%, especially in your likely salary range, it gets difficult quickly to find money to save, or to ever do anything fun (which is a necessity).