i have always tried to have good communication with customers- if 'life' happens and i get behind i right away contact the person and talk to them about the delay- if it's going to be a problem i will find someway to get it done on time- usually (because i do stay in touch) it is not a problem.
turn around definitly depends on the quilter- i know a couple who have 6 months to a year---i thought that was pretty crazy---i would not want to be responsible for someone's quilt that long- my (norm) is 2 weeks- but there have been times (last spring when we had 3 family deaths in a week) that set me behind- and i really had trouble getting back into it-and getting them done- i think one took me a couple months---but i did stay in touch - and make sure it was not a problem..
i think having good communication is a must. if you are happy with her work- but unhappy with her turn around- you should sit down and talk to her about it- maybe she could be more (realistic) about her time management and do better. or maybe it is just a hobby for her- and she has lots of family commitments- and simply does not want to be (scheduled) if that's the case you could find another quilter- or plan accordingly- but communication is the important part.