When I was little we lived across the road from my paternal grandparents, so I saw them most days as I would call at their house even if my parents didn't, or they would be in the garden or the street. And their back garden shared a fence with my school so I could talk to them at break times if they were in the garden.
We saw my maternal grandmother twice a week, we would visit her on Wednesdays after school and alternate Saturdays we would go to her house or she would come to ours. She lived about 15 minutes away on the bus with some very busy roads between us.
LO sees my parents about three times a week, because they babysit while I work.
PIL's are estranged at the moment so we don't see them. If things were normal I wouldn't object to taking LO to see them once or twice a week, (which I suspect would still not be good enough for them given the issues that we have which have helped to cause the estrangement). And it wouldn't be a fixed "we see you every Tuesday from 2pm to 6pm" or anything, more of a "are you free this afternoon, we will pop in/would you like to come over" kind of a thing that varied. As they don't work or have other commitments this flexible approach isn't impossible.
But I don't think there is any such thing as 'normal' when it comes to families and you would have to consider a lot of things. Who works and when? Who lives where? How well you get along? Who comes to whose house? How long you/they stay? Is it convenient for everyone? Would you drop the children off and leave or stay and visit too? Would you like them to come to your house while you go out and do the shopping in peace etc? Are they happy to vary the day they visit?
Not every family will need or want to consider all of those and other families will consider other things I haven't thought of.
But I don't think it is too unreasonable for them to see your children once a week as long as you can all agree on how rigid or flexable the arrangement is and as long as your DH is happy to do his share to make it work for you, your children and his parents.