Two weeks is usually sufficient time for fermentation to complete, but you will NEVER know for sure without a hydrometer. When the reading on your hydrometer is the same for a span of 3 days, fermentation is complete. If you bottle before this point you can have bottle bombs ! Plus, if you don't let the beer finish fermenting it just won't taste as good. I always plan for a 3 week fermentation for "normal" gravity beers, and for the "big" beers 4-6 weeks.
It's tough to wait, but it's amazing what just one extra week in the fermenter will do for the quality of your beer! Patience is a virtue, and in brewing it also seems to be the path to great beer.