How long to take off work to look after my mum?

My mum was diagnosed with terminal cancer last Autumn.....she has deteriorated a lot these past few months and a week ago was rushed to hospital. She almost passed away then. Work has been very understanding with the situation and have said that they can give me paid holidays for my absence. My question is, it's coming up to the end of my second week off, my mum isn't much better....do I request more time off? I'm scared to leave her....in case something happens but at the same time I know I can't stay off work indefinitely,....what should I do?