1) I was wondering how long do regular brake pads last on an average?
There are so many variable to give a good answer to the phrase "average." How do the brakes feel to you when stopping and do you notice if your stopping distances are getting longer? The brake sensors built into the pads will give you a dash light indication when the pads are getting thin. I would have your brakes checked annually, since it's good to check the rotors and the brake hose too.

2) I can feel my engine running sitting in my car. It is a mild vibration. Is it time for new motor mounts?
If they're not been changed already, they would be suspect.

3) What are the symptoms of my flex disc going out?
Remember there are two flex discs, so have them both looked at too on an annual basis. Flex disc wear will cause the prop shaft to wobble, causing vibration and possibly some contact by the shaft to the chassis. You'll hear it.