I'm so sorry about the diagnosis, I hope that COL can help you both through this horrible time.
As others have said, you will just know, you will no longer be asking yourself if it's time as some how you will know.
One thing I will say from past experience, Humbug dosn't know or understand the diagnosis, he is just feeling unwell so it's our job to fuss and spoil them in this time and keep things as normal for them as possible.
No one told my cat that he had 6 months and he lasted a lot longer then anyone expected, we just never know how much time we have with them