There are a couple of ways of guessing if your brew is finished fermenting but the best way to tell for sure is by using your hydrometer.
Most cans of concentrate have a hydrometer reading you can bottle at listed on the instructions on the can. This is fine if you are using dextrose and their yeast but using different sugar blends or different yeast will make this reading wrong.

People often bottle their beer when the airlock has stopped bubbling and sometimes this is correct but in some cases the airlock bubbles occasionally even though the yeast has finished its job, so the beer ends up in the fermenter longer than it needs to be.

By taking two hydrometer readings 24 hours apart you can establish if the yeast has done its job and make sure your beer is bottled at the right time.
When you think the beer is ready to bottle take a hydrometer reading and write it down where you won?t lose it.
Take a second reading 24 hours later. If the two readings are the same you are ready to bottle. If they are different wait a day or so and try again till you get the same reading twice.