Hi OP, so sorry to hear your news. My mum died a month ago from metastatic breast cancer that had spread to her bones and liver, so although not quite the same origin of cancer, probably a similar pathway.
Everyone is different but I can let you know how things went with us to give you some idea.
My mum was actually pretty healthy and was quite positive about her chances of living quite some time, but in the space of a couple of weeks the tumours on her liver became very aggressive. The last 24 hours were very quick - she went into hospital in the morning with some pain, and it turned out her liver was basically giving up. She was retaining fluid, but didn't want any invasive treatment at this point, so she was given painkillers and made comfortable. Originally we thought at this point she might be sent home and still have a few days/weeks, but as the fluid retention worsened, her breathing became worse and she had to go on oxygen. It was at this point that we entered the final few hours stage, and the doctors were quite clear with us that she would not get any better at this stafe.
She stopped being able to communicate around this time due to the medication. Her breathing was quite noisy, but the doctor said that this wasn't a sign of distress, just to do with secretions and the air passing over them. We felt she was becoming uncomfortable towards the end of the painkiller 'life', so we asked them to up the dose to maximum. When we did this, she settled into a deep sleep and her breathing became quiet but still steady. The doctor said that we could 'speed things up' by removing the bag attached to the oxygen mask that was doing a lot of the breathing work for her. We knew she wanted only palliative care, she had been quite clear, so we agreed to do that. Her breathing continued steadily for a couple of minutes and then got a little erratic, not noisy or panicky, just a missed breath here and there. Then she took a breath out and just didn't take another one in. About 60 seconds later she took one more breath and then no more.
We sat with her for a while before letting the medical staff know, and they came and disconnected all the medical equipment and then left us to sit with her for as long as we wanted. Her skin got a bit mottled after a while and she started to get cold, and at that point we decided we should get home (it was 4am) and the doctor came in as we left to do the various checks to declare her dead.
Sorry if this is a bit long and too much information, but I had never been with anyone at end of life before and had no idea what to expect, and actually I found it less traumatic in some ways (it was peaceful) but more traumatic in others (I keep getting flashbacks to being in that room with her after she had died).
If your mum is lucid and able to communicate etc just now, I would get some practical stuff in order if at all possible. Try and talk about funeral plans, it's awful but it's even worse trying to plan a funeral when you don't have any idea what the person wanted. My mum left instructions, which made our lives so much easier and we knew we were doing what she wanted. Make sure you know where financial information, birth certificate, etc. is if possible.
My mum had a copy of this on her laptop. I haven't read it all as I found it after the fact, but the fact she saved it suggested she found it useful: https://www.royallondon.com/about-us/how-we-are-run/mutuality/how-to-die-well/
There is a Before You Go section which has practical and emotional stuff and also stuff about dealing with things after death.
On an emotional level, use the time you have now to say whatever you want to say. Don't wait until you think it's the end. We didn't think it was the end and it turned out within hours that it was, and there are things I would have liked to say to my mum and conversations I would have liked to have had that I wish I'd had earlier.
This might sound weird, but make sure you have recordings of her voice. I was v upset as I thought I didn't have any and would forget what she sounded like, but then I remembered she had sent various videos on WhatsApp for my young DD, and I was able to retrieve those and back them up so I will always have a record of her voice.
Use the time you have to spend as much time with each other as possible. Leave no regrets about things unsaid or time unspent. And if you are with her when she passes, as horrible and upsetting as it is, it's also a profound privilege to be with someone when they begin their journey onto the next phase of whatever there is, whether you are religious or not. I'm not, but I found that it was quite a spiritual thing almost, as well as being deeply, deeply sad and difficult. Take the time to be present in the moment, don't panic or rush to get the medical staff - there is no need to hurry in that moment. As she is passing, take the time to recognise that you are with her, that you are together as she is embarking on the next phase of her journey, whatever that might be, and allow yourself time to sit there and take in the moment. There's a shitload of stuff that comes after, but it will come regardless, so take the time when it's happening to be unhurried.
Sorry, this is extremely long, but it's fresh for me and I hope some of it at least is helpful. I'm thinking of you and your family, it's an awful, horrible time, but make the most of the time you have and leave no words unspoken about your feelings.