the immigration law got changed, you could leave canada more than 180 days but in order to maintain your Permenant recident, you will have to leave for total 3 years out from 5 consecutive years.
eg you landed in 2006.. then upto 2010, you will have to have an accumulated lived in for 3 years. these 3 years could be - a 6 months in 2007, whole year in 2008, 3 months in 2009, and another 3 months in 2010..
hope this answer your question