Brakes Help

[TXtwowheeler]

The front brakes on my GZ 250 make some noise when I'm rolling without the brakes engaged. When I engage the brakes, they stop firmly, and they don't make any noise. Then the noise quits for a while, and then comes back again. It's not very loud. It's just the pads rubbing lightly on the disk from time to time when the brakes are not engaged. I can hear it occasionally at low speeds. The disk is not grooved.

I have 6,000 miles on it. The thickness of the pads is a little more than a nickel, but a little less than the thickness of a nickel plus a dime. On a car it would be time to change, but I don't know how thick the pads on a bike should be.

As always, my issue is in multiple parts:

Part one: When is it time to replace the front pads?

Part two: I've replaced pads on cars many times. Is it significantly different to do it on a bike? It looks pretty simple, but is there any surprise waiting for me?

Thanks guys, for all the sound advice to a novice biker.