I am currently on an F-1 visa and I have been in the US as a student since 2012. The only year that I have submitted a tax return because I was employed by my school was 2013. The country that I am from has a tax treaty with the US, and I am wondering what I should write in the field "Number of monthsclaimed in prior tax years." Is it just the number of months I have worked in 2013 or it has to be for the entire year of 12 months? For what it's worth, I left the US for a brief time in the summer of 2013 and 2014.

Enter "12" months.

Your absence in 2014 needs to be reported on Schedule OI. Other than that, it does not affect your tax return.

So, I should enter 12 months even though during that year I've only worked for less than that? As far as the report for absence goes, I already have specified all dates I entered and left the United States.

Okay, then you should be good to go.

Thank you for your assistance! I believe that the tax treaty can be claimed for up to 5 years and they are keeping track of that/making sure that I report it correctly, is that right?

Yes, it is five years for F-1 visa holders.