Hi Ruth
It all depends on what you want to achieve with your running. If you are just starting out then do as Nick explains set yourself small targets.
The important bit is not to go at it like a mad woman - that way the only thing you can guarantee is an injury. Add to whatever level you are at by no more than 10% per week, you may need to actually drop back some weeks if you feel tired before again adding more.
I personally ran my first marathon on 3 runs a week albeit very focused runs. See how you feel as you add to your schedule and always listen to what your body is telling you.
Good luck and enjoy.