if you want, you can drain and refill every 5 years, mostly because of chemical buildup. water cost is negligible compared to chemicals (chlorine tabs, filter material, water balance chemicals ...) of course, electricity costs about $1-$2 per day if you have a powerful pump maintaining a swimming pool is a lot of work, especially for someone who has never used one. If your landlord asked you to maintain it, I suggest you walking away from this house. You don't want yourself to get into troubles.