is it fair to give my 23 year old step daughter six months to move out?

a month ago, my 23 year old step daughter came to live with my husband, my 11 year old daughter(from my prior marriage) and me. she has a horrible track record on keeping a job. i've helped her find a decent paying full time job with full benefits.she starts oct. 19th. we are not going to charge her anything to stay with us.my husband and i feel that it gives her plenty of time to save enough money for her own apartment. i think she's immature for her age and will try and live with us indefinately. i won't be able to tolerate that. i'm not one that can adapt to long term stays from others, no matter who it is. what's the best way to let her know where we stand on this?