How much freelance work do you normally do?

On average 3-4 hours a night. That can differ depending on life's circumstances however.

Do you find that there are a greater number of American sources of work, as opposed to Canadian, that are available to you?

Most definitely. 90% of my work is from the United States.

Is there a difference in working for an American company as opposed
to a Canadian company?

Aside from benefits and pay that would be subjective to the company itself, not really.
 
What kind of experiences have you have working for American companies?

I feel I have a positive work relationship with all of my American clients. it really boils down to good work ethics and meeting deadlines.