Yes CBSA at the boder will decide how long you can stay. If they enter no stamp in your passport then can stay upto 6 months otherwise they may stamp your passport and give you a date by which you must leave
I have a similar case here in which I received a passport stamp for leaving Canada ( arrived on work permit), but I think that this is due to passport expiry. If I extend my work permit, can I leave at a later date?