Sorry, 我記得以前好似唔駛的. Anyway, if you want to get the visitor visa from HK US embassy, normally , will get 10 years. But you may need to provide travel ticket and a lot of information to fill up the application form first.
better apply your US visa while you are still working in hk and normally they will give you 10 yrs visitor visa. otherwise it would be hard to get one in Canada without employment letter.