I asked the same question when I made my first brew, someone said its usually within the first 24 hrs, but the fermenting bins arent always completely air tight so if it doesnt bubble dont worry. i was advised to give it a good stir after 24 hrs then after that the residue formed around the top of the waterline, ive since bottled that beer on sunday there after it was sitting for 2 weeks and it came out at 1008 so it has fermented.
ive got a coopers cerveza fermenting and it hasnt bubbled as far as im aware after 1 week, unless it happens when I'm at work! I made up a coopers lager kit last night and its yet to bubble, my fermenting buckets can't be air tight
someone said on one of the other forums airlocks are for winemakers, u can leave a section of ur lid unsnapped and that will let out excess co2