Salt Lamps are natural salt and are therefore intended for indoor use, not outdoors. Keep them away from water. If your lamp accidentally becomes wet on the outside, turn it on and let the warmth of the bulb dry it out. If it becomes immersed in water, do not turn it in due to risk of shorting out the electricity in the bulb. It is safe to leave the lamp on all the time to enhance the energetic field of any room.
Salt lamps are not "used up" by being left on. Salt does however have a tendency to absorb moisture out of the air. If your lamp does "sweat" simply pat it down with a dry cloth or paper towel and let the warmth dry up the remaining moisture.
If you intend to store the lamp unused for a period of time, place it in its original box or plastic bag and store it in a moisture-free environment.
It is always best to place the lamp on a protected surface since a salt lamp may sweat at various times.