I had travelled to canada while on OPT. My international advisor told me that I can be out of USA for less than 5 months only.
i was only gone for a week so i did not ask any more questions regarding this issue,

@ the border they cop looked at my passport and i20, asked me if i have proff of sevis registration, when i said no, he made me wait 10 mins and let me go without any more questions.

thats all i have to share, lets hope someone out there knows.