“Excessive” is when you begin to feel resentful and/or used. That will vary from person to person, and family to family.
In my culture, grandparents help to raise children. They do this while the parents work, or they work outside the home so a parent can remain home. It’s more common to live with extended family for the benefit of the extended family. What you describe would be normal … for me, for us. And in fact, it IS normal for me; my mother has my children every day that I work (my job requires international travel). We share a home.
My husband’s culture is also very family-oriented, however they don’t live as an extended family. It’s not uncommon for grandparents to be very present in the lives of their grandchildren, but caregiving is more often compensated and negotiated. My husband’s grandmother came to the house to care for him every day for most of his young life, but she was on payroll (taxes and all). There was an understood contract and terms that all parties were satisfied to uphold. What you describe would be unacceptable without a contract, implicit or implied, … to him, to them.
What I’ve noticed, as an adult living in America, is that today’s (Western) parents seem to want the benefit of the first arrangment without the responsibilities of the second. Not all, just some. I’m in my mid-30s, and this is what I see of those peers not sharing my culture (or the few others that seem very similar to my culture in terms of family roles). I can see where a grandparent might feel resentful. I actually DO see it in my husband’s family.
My SIL has two young boys (ages 9m and 34m) who stay with my MIL 12 hour days, 5 days a week. My SIL & BIL meet at MIL’s around dinnertime to pick up the boys and to also be fed themselves. My MIL and FIL own their own business, which is labor-intensive and must now be scheduled around their (uncompensated and IMO excessive) babysitting duties. SIL & BIL also are big on weekend getaways, scheduling at least one monthly to out-of-state locations without the children. My MIL keeps the kids these weekends, too.
Is it excessive? My husband and FIL think so. My SIL and BIL obviously don’t. My MIL feels it is but will never say so to my SIL for fear of alienating her and losing the kids entirely. Or, worse, as other posters have suggested, for fear that the kids would then spend the same amount of time away from family in the care of a “stranger” … so she sucks it up, and deals.
I think it’s excessive because MIL feels put out, and because it’s affecting MIL’s source of income. But if she’s going to allow it to happen, … well, there’s no right complainin’ then! We can all only hope that as the kids get older, SIL feels less overwhelmed and is better able to relate to them and to BE PRESENT for and with them. Until then, MIL feels she’s best able to provide that for the grandchildren. So excessive? Who’s to say, but you …