Can I ask my supervisor how long would it take to complete my PhD before beginning my PhD studies and post getting acceptance letter? Can I ask my supervisor how long would it take to complete my PhD before beginning my PhD studies and after getting my acceptance letter? If so, what would be the best way to do so?
Country: US/Canada
Duration: It is not open ended like Europe. Depends on Professor here.
This question doesn't really make sense, the supervisor has relatively little control over how long it takes for a student to produce a thesis.  Questions you could ask which do make sense:

How long is funding guaranteed, and in practice what happens when students run out of guaranteed funding?
Do you have any additional standards or requirements for a thesis, such as requiring it to include an accepted paper, or requiring it to consist solely of singly-authored material?
How long have your students typically taken to graduate, and what factors played a role in that time?
How does a students career choices effect your advising process?

(The reason the last point is relevant is that if you want a research job it's quite simple, if you have good enough research to get a postdoc and you've actually written pages then you'll graduate.  But for a student with teaching or industry career goals there's more flexibility in trying to get a student out more quickly with a more minimal thesis.  Some advisors may be more understanding about a student's prioritizing getting out quickly for career reasons rather than produce the most compelling thesis.)