You have to decide what's right for your location/situation, but if you have a good rapport with the guests and want to extend, why not? You can always try and get a bit more info about their future plans. Are they moving to your area for a job, or just a longer vacation? Do they have residency papers? Do they have family nearby? Anything that you can find out will be helpful in making your decision. If they are neighbors of yours that got kicked out of their house and are trying to find a new place, then I'd be hesitant. But if you have a good feeling about them, why not?