There is not guideline like do it for so many weeks and than stop, it's a judgement call you have to make based on that specific animal, size, overall health, body condition etc.if not how long should I try to convert her to rats before I give up on it?
If it was my animal at that size I would do 5 feeding using rats than on feeding number 6 if I still get a refusal with rat I would offer a mouse than I would do the same again, and again and again.
Best time to switch is when they are young however they need to be hungry too and with the amount of food you have fed recently it will take time if it is meant to happen, not all of them will switch and some may much later on in live after months or trying.