Greyduster 
I ring my Mum around 5 o'clock every Sunday for a chat, but she has no problem ringing me if I'm late.
That's our regular chat time, but we will still speak several times during the week, just not for as long. She lives over 300 miles away and we both like checking up on the other, to make sure the other is OK. It also means we know what's going on in each other's lives which suits us as we're close.
My FiL usually only rings us if there's a problem, so a call from him can be a big deal, but he's insulin dependent diabetic, so we ring him every few days for a little chat. He's very independent and can be a devil sometimes, so we like to keep tabs on him.
My Dad died 18 years ago, but I could count on one hand the number of times my Step-Mum has phoned me. I stopped ringing her years ago because we never had that much to talk about. We live half an hour apart and get on fine, but some how we don't keep in touch. I think if she'd made some effort, then things would be different, but I just felt I was making all the effort and getting nothing back. We send birthday cards and catch up at Christmas time when I go to see her, but otherwise her life revolves around her family. I just hope if something happens to her that her family let us know.