I’m very sorry to read your story OP, I know exactly how your DH is feeling as I went through it with my mum last year. End of life care and the dying process all varies patient to patient. Unfortunately there is little resources available to loved ones about how to watch someone die and what to expect.
I was completely blindsided by the experience as I am sure your DH and you will be. I always wondered why the nurses at the hospital my mum was in kept telling me and my family to look after ourselves and get rest as much as you can. Mums final week was round the clock bedside waiting for the inevitable to happen. It was exhausting physically and mentally you really do have to make sure you care for yourself.
So priority number one is look after yourselves, your FIL is coming to the end of his ordeal and the medical professionals and hospice staff will ensure he is stable and pain free, let the burden of care fall to them. Once they stop hydration and feeding then it’s simply a waiting game, it’s a surreal experience, those hours sat in the room by mum watching her die truly felt like I was in Hell itself. Ask lots of questions to the staff, raise any concerns you have, ask for my pain relief and sedation if necessary, they will be as accommodating as possible so do not hold back.
The final breath was a very serene calm moment and within seconds the empty shell on the hospital bed was no longer my mum, so take comfort in knowing that death is really just a step we take into another place wherever you believe that to be.
The relief after the suffering was over was joyous, it was like quenching a thirst you’ve had for days. It’s a shit situation to be in but so many are going through exactly the same as you right now. Please tell your DH to look after himself and that everything will be alright.