I'm not sure the timetable of when they knew Christine wanted back in -
but I remembered this post awhile back that came out in October discussing Stevie's desire to do another solo record soon.
http://www.fleetwoodmacnews.com/2013...-nicks-is.html
So we know that Stevie has been planning this for a little while now.
Also I swear I remember reading something more recently that Stevie hinted at a couple different upcoming FM tours. Couldn't find it though.