Deductions & credits

I have a question. I think you have already answered it but I am a little confused. 

I am a US citizen. I started working abroad July 17th 2018. I continuously met the 330 day out of country and have been exempt from paying taxes. My employment may end either November 31st 2020 or March 30th 2021. Am I required to have 3 block years (12 month periods) to be tax exempt? If I only worked 2.5 years abroad or just came short of the 3 year mark, would I owe taxes?