To answer the "how long?" part of the thread title:
I think that unless there is a marked improvement, she'll go next week.
If they do actually look better as in perhaps a simple routine with Brendan not dragging Fiona around the floor during the Foxtrot, then I'm afraid it'll be Dennis or Will facing the chop unless they do very well with the judges.
To be honest (andI have no teaching or dancing experience so you can dismiss everything I say), I think if they pick a very simple routine with basic steps and do them well, have good posture, keep in time with the music, if the outfits are beautiful and they choose a good piece of music (perhaps something more traditional rather than a modern reworking), then the judges might appreciate the routine and they'll get through.
I think that from ITT tonight it looks like Brendan has reevaluated his goals and is going more for the keeping-things-simple approach and try to make it fun for Fiona, which will be fun for us - we might see something like the bum-plonk at the end of the jive!