A good friend of ours has relocated jobs back to London, and we offered our spare room whilst she flat-hunts. This was about 3 weeks ago. She buys food and cooks once a week, and offers to babysit also around once a week, which is nice. She's also rarely around at weekends. However, no sign of flat hunting or mention of an end date. How long before you'd ask her her plans?
I'd ask now before it becomes too set in stone and harder to approach. Could you maybe say that a family member (eg Grandparent) wants to visit soon (and needs the spare room) and does she know when she will be in a position to move out so you can sort out dates with this other person?
A friend of ours moved in with us following a relationship breakup... He stayed for two months rent free but of course contributed to food and cleaned up etc. We decided to make it permanent and he started paying us after the two months.
Having just got rid of my SIL who was with us for 10 months (!!!!) I would say you need to address this sooner rather than later. We agreed she would be there for 6 months and I had, what I thought, were reasonable expectations that she would (a) help out around the house (b) babysit regularly (c) cook occasionally and (d) pay something towards her keep. In the end she cooked about four times in 10 months, never helped with any cleaning (but did complain that the cleaner had not done her bathroom properly ), babysat about five times and didn't contribute one penny of rent or money for food (i know!!) or electricity or gas. She did pay for two pizza express meals and about 10 breakfasts out in a local cafe, but by the end even my DP's patience was wearing thin. So to sum up, set the ground rules now before it is too late and it becomes an "issue".