We maintain several zones of personal space depending on our comfort level. We will let anyone, other than terrorists, to remain more than five feet out of range. We are effectively ignoring these people. Of course, this range may be used for distance flirting.
In a business or social setting we expect people to remain two to four feet away to maintain a comfort level. This is where you need to be in the initial stages of flirting. When signs of success are obvious and you think you are invited, you can move into the intimate zone - an arm's length or less. In general, single women will not allow anyone this close unless they are very comfortable and very interested. If you get to this point it is a very good sign. Do your best to stay here as long as possible.