Please fill out the number of months you’ve spent in the US and claimed a tax treaty on your prior tax returns. If you are unsure whether or not you've claimed the tax treaty before, you can check Schedule OI on your previous Federal tax return - Part L.
It is possible that nothing is written there though and if that's the case then you can count and write all the months you worked before the current tax year (you must not include the months during the tax year you're filing for now).