Stripped threads on left crank on the first ride I just purchased a new road bike and had a friend assist me in the assembly. There must have been a problem because it fell off after only 2 miles of riding for the very first time. I used a pedal wrench and tightened it. The pedal was indicated as being the "left" pedal and has reverse threads. 

Now the threads on the left crank are about half stripped. I have successfully screwed the pedal on but am worried about it failing now. The crankset is a Shimano FC-RS500. I would post a picture but I don't want to do anymore damage to the threads by taking it off again.

Do I replace the left crank now or just wait and see?
First thing to check: the left pedal uses a left-handed thread. Was it screwed in correctly? Depending on the amount of damage to the threads, it may be possible to sort-of screw it back in the wrong direction, which will damage it even more.