On the job training

I started working part time at chiropractor's office last month with no previous experience in a medical office. He has one other office employee who has been with him for more than 10 years (she works 1 day a week). He's been through several other employees recently. I have 15 days on the job - the docotor is training me in between seeing his patients. I definitely feel as if I'm grasping the work, but he is upset that I still have questions when it comes to applying payments or concerning EOB's,and wants me to just understand it right off the bat. It doesn't seem unreasonable to me that I have questions at this point, or I'm unsure of some of the processes. The massage therapists in his office make comments about his difficult nature. I'm wondering what is the average length of training time for this type of work? Any feedback would be helpful.