Hello everyone. I've had the MM for about a week now and I've gotten a feel for how it flies. I'm really impressed with it. That being said, I made the terrifying mistake of nicking a tree branch in my drone recently (over a river too good lord), though it remained stable, if spazzing in the air for the brief time it made contact. Upon closer inspection of the drone, there is a slight mark, not even a nick, on a pair of my propellers that had hit the aforementioned tree branch.
This brings up the question: How often should I change my propellers? How can propeller damage influence a drone's flight characteristics? I feel like my propellers are still in good condition, but this event really made me wonder about how long propellers can last before they need to be changed.

Hi there, the propellers should be replaced at once whenever they were bent, scratched or damaged. Since your aircraft has hitted the tree, for the fly safety purpose, if there is any abnormal with the drone, you are recommended to contact our support team via the link below to open a case and send the drone in for a thorough diagnosis.
http://www.dji.com/support
If there is anything else we can assist you with, please feel free to let us know. Thanks.

MM props are so thin and fragile that I would not risk it: if there's a mark you can feel with your finger or nail, I'd recommend you replace them. Over-cautious maybe, but for the price, I'd still do it.
The flight controller is capable of adjusting and coping with damaged props (up to a certain point, of course), so your AC might still be flying pretty much like before. Typically, damaged props become unbalanced, so the first thing you notice (hear and/or see) are vibrations. As for how long they last... in normal conditions, if you don't crash, they can last  for a very long time (forever?). Just keep an eye on them, clean them occasionally (Windex works well), check for signs of wear, hairline cracks, excessive play by the screw, odd shape (compared to new)... and if something is not right, replace.

Question related to this - has anyone currently located a within US supplier of props that can send? I know things are hectic right now and I'm sure DJI is backlogged over there but does anyone keep stock of props aside from them?