How many hours do doctors work a week on call?

This depends on a couple of different factors. First, is the type of physician. Surgeons work more than anesthesiologists (usually). Family practitioners and Internists work more than Emergency physicians (if the FPs and Internists take call), etc. Second, is the type of practice. Some docs have only clinic hours and do not take any call.

In residency (where I am now), there is an 80 hour a week limit and no more than 30 hour in a row limit. Once out of residency, there is no limit on the numbers of hours worked and you must work until the job is done.