How long does it take to get an Invitation letter from Townhall for France visa application?

I'm an Indian girl. I am planning to visit my sister in France. My flight leaves on 22nd july 2011. My visa has once been rejected. I'm re applying. I need an invitation from my sister which she has to obtain from the Town hall[(Attestation d'Accueil (Certificate of Board and Lodging)]. Can you tell me how long does this procedure take ? so that I have an idea as to re applying is possible or not, since my flight leaves on the 22nd, less than a month left now. Thanks!