How long until I need to replace...

[justben]

... my pads and rotors after the pad wear indicator light comes on? I looked at all 4 corners last night after it came on and the pads LOOK like they have plenty of life left on all 4 corners. Obviously this is only the outside pad not the inside. I was trying to pinpoint which to replace, front or rear, but they all look pretty much about the same. Please tell me that I don't have to replace all 4 corners at once.