What car, what year. It could be a leaking hose or seal or pinhole rusted steel line on the steering rack. If you have to top up the fluid frequently, you can use Lucas power steering stop-leak, which might fix it, or just slow it down, and cost the same in the long run.
I've driven a car with no PS fluid for a couple of months, just adding a splash now and then to keep the pump lubricated with no ill effects. The steering will be harder, so it may not be fun. If the car is 10-20 years old, it will probably need a rebuilt steering rack if it's not just a hose.