If she wants to talk for an hour but you only have 5 minutes (and assuming it's a coherant conversation, not with Alzheimers in the mix), try to make the hour for her. And, if you need to, tell her you love her each time.
I believe in this life it's the last moments we do, rather than the whole of our life before that. The whole of her life may have been incredible for her, but ultimately in your present. So if you haven't told her anything yet or asked her for anything yet, follow her cues, as slipsidingaway said, and take advantage of signs.
Hopefully if the cancer has not spread, you may not be at that point yet, but start the process because at some point it'll be important but maybe not possible later. It doesn't have to be in a "you're dying" approach, just "Mom, we're both getting older and I'd like to talk about some of your life I don't know about, if you want to share with me. For instance, when you were in WWII/Riding Dolphins in SeaWorld, traveling through Asia, or just what you remember about my birth and what my life was like." Particularly if she'll talk about HER parents and HER growing up - we just don't pass that along any more.
It's good to make a point to talk to people I've wanted to, if they want to hear it. That's me. Others may prefer not to talk. Family legends and stories - try to remember it and preserve it.
Oh, and if she's in the mood and likes to go through photo albums, it's a good time to hear those as stories. My mother has all kinds of pictures (unfortunately a lot got stolen unexpectedly) and I don't know who any of these people are. Be nice to write it on the back in this day and age. Unfortunately when half the stuff is digital nowadays it becomes harder to pass these things along.
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