Dying doesn't happen on a schedule. Much like birth. If you don't want to have regrets, you just keep in touch with her according to her needs and your ability. For instance, she's not alone now, so I wouldn't think she would need daily contact, but if you get along with her you could call her once a week. Ppl that age appreciate being able to know when you're going to call. As in, it's sunday! Sue will call today!
the point isn't to say Good by just before someone dies, the point is to have a good time with them, enjoy them, if you do, and spend time with them when you can, and to make sure they're no alone and needing something. Then when ever death chooses to come you won't have regrets as you will have loved them, and enjoyed them.