Hi, really sorry to hear about your Dad. I can identify as my mum 72 has a GBM and she is one in my life I've always been able to chat to and receive empathy and support. I understand your main anxiety for your life and obviously you don't want your dad to suffer, it's so so hard..My mum was diagnosed with a GBM May 2016, she asked them specifically not to give her a prognosis, it was her way of coping and seems to have helped her mentally fight it. She had all SOC care treatment including surgery which is gold standard front line treatment if you can. NHS can be very reluctant to be pro active with this disease as they have weigh public spending with chance statistically of a positive outcome. My mum GBM was stable for a long time, but unfortunately returned this past summer (more severe then before). Again no prognosis wanted and mum is still very much a part of life and doing well in herself. She has always focussed on life through out this...my little girl see had seen grow is now 5. NHS reluctant offer active treatment now as probably less statistical chance of success in their eyes. We are waiting on regular hospital's decisions and if they not offering or recommending active treatment, or another op we will contact another hop that suitable & get a 2nd opinion. She is already had Temodor/TMZ chemo which well tolerated and helped keep stable with radiotherapy, but chemo tried again and not working now. I am also currently trying get her referred to private 'Care Oncology' clinic in London (you pay but set amounts many people can afford) as they offer evidence based re-purposed drugs that can prolong & enhance life & are well tolerated. Joining FB groups 'Brain tumour support groups and an American set up but international group 'GBM survivors to thrivers' been very helpful & inspiring. Mum takes lots of supplements which help & would recommend especially to anyone not currently having active treatment, there are special diets can help too. NHS do tend give worse case scenerios prognosis therefore even without treatment (it's your dad choice, but people do live long term with this condition but only if they pro active & have treatments ) ..your Dad could well be here lucid for months not weeks. I'm not sure why chemo (when generally GBM front line chemo well tolerated & radiation, not been suggested as you not mentioned.. I wonder if your dad understandably feels depressed after shock diagnosis & declining treatment as unusual be offered nogthing even 'pallative chemo'. My  thoughts with people's declines I'm not a medic or God, from reading people's posts over last few years who sadly lost their loved ones seems around a month or so in general, but every one so individual and very occasionally people do pass on very quickly as happens some people without condition..a Grandad was watching his Grandson play & smiling at him having lost some his verbal communication capacity & passed on soon after one story I heard..More often over a period of weeks, every family has said peacefully. The disease gives anxiety & little peace, but if patient is pro active you can make more memories with your loved ones. My heart does physically feel what you feeling with anticipatory grief, I've had it for years, none our parents are immortal, it's the hardest fact to accept. This disease highlights this in most direct way...we will survive this (I tell myself daily) and while they are here, it's helped me knowing I'm supporting & helping someone who been there for me all my life. Look after yourself. Much love to you.