For histology, the samples should be refrigerated at 4 0C, ratio between sample & fixative size should be 1:40 at least, then it could be good for at least 3 weeks, if rats were perfused by NBF, the sample would be stable for a longer time. Very good results usually within the first week of fixation.I heard from my colleges that samples could stand for a year, but when I tried to store the samples for only one month, they were not good and showed slight degeneration.
I have heard that some researchers kept samples in NBF for months or even years and still managed to get fair results on IHC. None of them used bone though. Most people I have talked to would prefer to stick to the traditional 72-hour rule but it is interesting to know if bone samples could last longer than that.