when do I replace brake pads?

[Psalm31.24]

I know the simple answer: when the current ones are worn out!
My problem, I don't know what worn out is. If new pads are about .5cm thick, i've got about .25cm left on the front pads. Is there a pin that will screech when I need to replace them?

 

[James]

There is usually a wear groove in the pad. When the groove is gone, it is time. I am sure they will screech, but I think it is too late at that point.

 

[mxchamp85]

I usually replace the pads before they screech because if they screech that probally means its metal to metal which can damage your rotor. Also, you can buy brake pads that come with a thicker pad to prolong the life of your brake pads.