how many months can F1 student stay out of country

I went to US on F1 in Sep 2014 and I left US in March 2015 and have been living outside US since then. Now I intend to travel back to US with i20 from a new university ( sevies transfer is completed ).

Is there some timeline before which I need to go back to US ?

My F1 visa is still valid so can I travel on the same visa ?

What all documents I might need at the port of entry ?

Your F1 status become invalidated, if you are outside of the country for more than 5 months. Typically, if you are outside US for over 5 months, you would need to go for F1 stamping again, even if the stamp in your passport is valid, as per new regulations. Read complete details on Validity of F1 status and stamping with break . I suggest you speak to the DSO at the new university that you transferred to immediately and take action as per their suggestion.