Let it ride and get a hydrometer. Bubbles in the airlock can indicate that fermentation is happening, but the only way to know that it is done is to use the hydrometer. Finished cider gets down pretty low, so your yeast probably have quite a bit of munching left to do before they are done. Even if you didn't measure original gravity, a hydrometer will still be handy. Your final gravity will probably be very close to 1.000, so when you see a steady reading in that range, you will know that your cider is done.