Too many variables. How busy are said adult children? How busy are the parents? How many of them work? How far away are they from each other?
I know several families who see their adult children every day (live a mile apart,) and others who see them twice a year. One extreme to the other IMO. BUT if people are looking after grandchildren, then of course they will see their adult children more.
We see DD every 2 weeks. She visits us (sometimes with her fiancé, sometimes alone,) once a month. Then we go see her once a month - so we see each other twice a month. She lives 20 miles away, and we do speak in between on whatsapp and twitter etc... DD works 4 days a week, I work 3, DH works 4.
I have found the parents who moan and whine that their adult children don't visit, are the ones who very VERY rarely visit their adult children themselves. They rarely ring either. They think it's the adult children's DUTY to visit and phone - especially the daughters. The sons are let off a bit more, because ya know boys will be boys..... 
My mum used to give me a right ear bashing if I didn't phone her every other day, and visit 2 or 3 times a week. Even though I was a very busy mum, with a daughter, and a job, and a home to look after, and she had no kids at home, and no job, and much more time to herself. She came to mine maybe once a year, and rang me once for every 20 times I rang her! And as soon as I picked up the phone she'd say 'I thought you were dead. Haven't heard from you for DAYS.' No asking how I was or anything, just an ear bashing.
I swore I would not be that person, and NEVER complain if DD doesn't get in touch for a few days. I will just fire a tweet or whatsapp message at her saying 'hellooooo , hope you are having a good week!' No reason on earth to make your adult children feel like shit for having the temerity to not contact you for 3 days. (Like my mum did.) Didn't do it to my brother funnily enough. 'well he is soooo busy and has very little time to himself with his big important job.' My job (which I worked hard at and was good at,) and my successes in life, paled into insignificance, compared to that of the flaxen haired blue eyed boy who was/is my brother!!!