Need advice on a Canadian visiting his American girlfriend!

Hi there! I have posted in this sub before and am always happy with how well people can answer travel related questions. I live in Toronto and visited my girlfriend in San Diego from august 28th, 2015 to feburary 17th, 2016. I returned home knowing that my 6 months I could stay out of Canada was coming close so I returned 11 days early just to be safe. I then went to Myrtle beach from March 13th-18th. I am wanting to make sure that I can visit my girlfriend for a week at the beginning of May. I have read and heard from my parents that the 6 months you can stay out of the country, are for the entire year. Is this true? Or is it 6 months every time I cross the border and am granted a new visitors visa.

Thanks so much!