I don't know if he would have taken the time to post if he was not serious.

OP, unfortunately, the standard is not whether the agency can "unequivocally prove" that the stone chip happened during your rental, as the poster above suggests. What matters is that if the damage was not previously noted, you can be held liable. This is why it is very important to always do a full walk around of the car before you leave the lot and if you see anything at all (dents, chips, etc.) to ask that it be added at the exit booth. Many of us take pictures and/or videos. I learned this lesson a few years ago when a stone chip I failed to notice (in the outside corner of the glass where it was black no less) turned into a full blown windshield crack during my rental.

In my experience it can take anywhere from a week or two to 6 months. If you filled out a form (not sure what you completed), the agency may cut you out of the process now and just deal with your credit card or insurance company.