Visa stamp at the port of entry

[Suin]

Hello,
could you please advise on the following situation. I was given a visitor's visa just for 3 weeks at Buffalo. But on the port of entry they crossed my visa and put a stamp that allows me to stay for 6 months instead of those 3 weeks given in Buffalo.
Could you please advise whether there's a chance in future to extend this stay given at the port of entry in Canada?
thank you

[accreddy]

Yes, you could apply from inside Canada to extend your stay as a visitor.

[Suin]

thank you for your reply. how many months before my stay in Canada expires I need to apply for the extension?

[accreddy]

You should apply atleast 1 month before its expiry (http://www.cic.gc.ca/english/information/applications/guides/5551E.asp).

[Suin]

Thank you so very much!