H1B visa stamp from Canada

I am a H1B visa holder and Canada PR. I plan to go back China in Christmas Holiday. I intend to apply H1B visa stamp in Canada before going back China. My questions are: 1) Is it risky? 2) Is the H visa has two times of entry to the US border? 3) Is the H1 visa valid for 3 months or six months? 4) Can I get the visa the same day or next day if approved?
Many thanks.