As I was told: if you apply at least 30 days before your work permit is expired you are fine. My work permit was till September 1. I applied for a extension of the work permit for the same employer in spring so I could get it before September 1st. I worried but it arrived a couple of weeks before September 1st. I know though if you change employers it does not take that long. I changed employers before and it took me about a month to get a new work permit for a new employer. I consulted CIC and Temporary Foreign Workers Office on this matter. They all said if you stay with the same employer and just apply for extension that there is no need to worry if you applied for extension before work permit expires. It is written also in the guide "Change, Extend condition of your stay in Canada" on www.cic.gc.ca