Per best buys website, it lasts through the next calendar year from when you meet it .So if you got Elite Plus status 9/1/18. You would have Elite Plus through the entire 2019 year.
What are the start and end dates of My Best Buy Elite and Elite Plus?
Your Elite or Elite Plus status begins about 15–20 days after you meet the qualifying purchase amount ($1,500 per calendar year for Elite; $3,500 per calendar year for Elite Plus). Your status continues through the end of the following calendar year, and ends on the last day of the following February. In any subsequent year, you may re-qualify for status when you reach the published spend requirement on qualifying purchases at Best Buy or BestBuy.com during the calendar year, or the spend requirement on your My Best Buy Visa.