A multi-engine add-on to a current certificate you hold carries no additional requirements other than those set out for the certificate itself. There is, therefore, no minimum aeronautical experience outside of the private/commercial certificate itself, and no knowledge exam to take.

The typical training time for a multi add-on is 10-15 hours of flight training and 5 hours of ground. This is en estimate and more training could be required

The ACS for the multi-engine add-on is contained in the ACS for the certificate to which you add it. Scroll about halfway through the appropriate ACS book to find the requirements and standards for your situation.