I am afraid it can take this long. My father was diagnosed nearly 2 years ago and underwent a large operation to remove part of the colon\bowel, a hemicolectomy. He was given 3-6 months but am happy to report he is fit and well and painting our gates at the moment, and he is aged 85! His first diagnoses was not made until he had actually obstructed. He had just received the results of his scan days before after a 2 month wait. He jokingly said, "maybe they were hoping I would snuff it and save themselves a big op"! They were adamant after the op that it had spread to his spleen, (very unusual in colon cancer) and then to his bones. Each time he has had a scan he has had to wait up to a month for the results. However they are so far wrong and no further signs of cancer have been detected anywhere. So, it could take up to 3 weeks for the results of the histology etc to come through. Plus it is the consultant who will discuss the findings with Mum, or one of his minions and cannot be given over the phone or by letter.
However there is also the chance that she is keeping something from you. Wait until the appointment and go with her if at all possible, it has been proved that only a few words of what is said to patients in these circumstances are absorbed, and they are usually in the wrong order! Try to get the time to sit and talk with Mum about her fears and express yours too. This is a time for support and understanding but only if she wants it. Give her every opportunity to be honest about her feelings, but at the end of the day it is her life and she must deal with this in the way that suits her best. Good luck with the appointment, I really pray that all your worry is for nothing, and if not do not despair as my Dad as proved every Dr wrong so far! Be positive and God bless.