I won’t lie it won’t be easy going. Remember she is still here and you still have time together. Enjoy the good days and try to get though the bad in the best way you can. You will have days when you just can’t or don’t want to deal with things. That’s ok your not super human and you need to look after yourself too. Your mum and dad will understand and know you still need some me time.
Take help from who or how you you can when you need it. No one can tell you how long you have try not to focus on that. Talk to them make good memories so when things get dark you have something good to look back on. Take the lead from her and your dad if she is happy and want to try and make the best of it then do it.
Above all like I said look after yourself and if you need time out take it. Only you know what helps you deal with things. If you feel you real can’t deal with it then find some help even if it’s a quick coffee or pint with a mate. It all helps.
Take one day at time or you will go nuts and start thinking all sorts and what if’s.