original entry date for visitor extension?

[Nexus2013]

My CAN citizen g/f and I have been going back and forth between US and CAN for 2.5 yrs with tons of border crossing and airport passport entries. For the last year or so, we have spent most of the time here in CAN. I have made many trips to the US since Jan, but my cummulative calander year time here in CAN is getting close to 6 months.
I'm filling out a visitor extension becasue I will stay here while my PR application is being processed.
Questions:
1) Is the 6 months I'm allowed to stay "reset" each time I leave the country or is it cummulative for a calendar year?
2) What is my original entry date? (which entry time would I use?)
2) When should I apply for my visitor extension? (The last time I entered CAN was 10/14/12)

Thanks in advance!

 

[Creampop]

Each time you leave Canada the 6 months resets... nothing is cumulative.
"The last time I entered CAN was 10/14/12"... this is your original entry date
you should apply no later then March 14th 2013