I don't think there is any 'right' answer.
One thing I do know is that the amount of time spent really can fluctuate depending on many different things. Obviously distance is one. Just as I expect my kids to decide where they want to live when they start a family, I hope they don't feel my DH and I should follow them wherever they go (or that we cannot move if we decide that is what we want to do when we have an empty nest)...with 3 kids, which place would we pick if they all move to different places?
The age of the kids is another. I know that my kids saw their grandparents much more when they were younger. Now that they are older, their lives are full of many more activities, so they see their grandparents less.
Lives (health) of the grandparents- My mother has gone through many different stages where she is very busy, or not so much. That definitely plays a role in how often she sees the kids. But they know she loves them so it is not a big deal.
I think the bottom line for us, in our family, is that we don't have expectations, we just do our best to make each other feel welcome in our lives. And we try to be understanding when the others are busy and don't have much time to visit. We don't leave it up to one side to be responsible for suggesting get-togethers. If my mom hasn't called for a while to get together, I call her. If I haven't called her for a while, she calls me. Same thing with the other grandparents. If we or they are to busy or whatever to visit, we just chat over the phone or email (as do the kids)