Ideally, it should be about 3-4 ft. However, it depends on the model and brand. In case you need a longer one, they are easily available to purchase. It is generally advisable to buy from the same company and not try and make your own (which is very possible).
Another thing to keep in mind is that a long hose will make it difficult for the AC to cool the room. As a general rule of thumb, a hose shorter than 10ft will allow the unit to function correctly.