its really not a good idea to run your tank so low. but, i am just as guilty of it as the next guy.
anyway, its depends on the driver, the amount of weight in the car, and a few other factors. driving habits are probably the biggest thing that changes this for every driver. i wouldn't get into the habit of running so low though.
if you run your tank empty, its a good possibility that you'll draw the sediment from the bottom of your tank into the fuel rails and injectors. over time, this will slowly degrade your car's preformance.
as mentioned before, its a good practice not to wait till you're on fumes to fill up.
just my .02...