Take a hydrometer reading once a day, around the same time if you can. If the reading is the same for three days then its done. Draw of the sample slowly so it doesn't froth into the sample jar, otherwise it will be difficult to read.
However, if the airlock is still bubbling, then it's a good indication that it is not finished. I have had brews that bubble for as many as 10 days (and as little as 2) and have turned out fine.
Give it a little time, let the yeast do its thing. Also remember to give it time in the bottles to let it mature otherwise it wont taste too great.