My mum and my MIL had my son, and then my second son, when I went back to work. They originally had DS1 for a day and half each. Then when he went to nursery, it went down to a day each.
When DS2 came along, they had each one separately a day each. So DM would have DS1 on a Wednesday and DS2 on a Thursday and MIL the other way around. This was because there is 3.5 year age gap and trying entertain them both at the same time was hard when I had a 4 year old and an 8 month old or whatever.
DS1 is now at school and DM collects him twice a week, has him for an hour or so until I collect him. I do the rest of the pick ups.
My brother also has a DS and my mum has him once a week too but the hours are such that there is no cross over, so she never has more than one boy at a time except for an hour on a Thursday when she has both my two after school which is fairly easy because DS1 is knackered and just lies on the sofa watching TV and eating biscuits!
DH's sister is never likely to have kids, so my MIL will never have any other GC than my two.
All the help we get was offered (demanded in my MIL's case - she hounded me for 5 months until I went back to work earlier than planned so she could have DS1 - but that's another story!) and never asked for. But it is very gratefully received! Should either set of grandparents ever feel that it it too much for them, alternate care will be found immediately, and they will just have the normal weekend visits or whatever works.