There are so, so many factors that go into that. While I'm sure you could generalise to some degree, a single case will be meaningless and the variance person to person would be very high (here is an example of some real data but is missing both of these countries: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10519902/)

The immediately obvious factors to me that would all strongly influence this would include:
How many commitments they really have (this might include work, volunteering, friend groups);
How much time & energy they already spend caring for someone else (particularly their partner);
How much energy/general impact of their physical health - e.g. is it easy for them to move around and chase kids;
The age at which the grandparents and parents both had kids will heavily impact all of the above (Quick scan/aggregate of Google images graphs for this suggests this has risen about 5-7 years total steadily since about 1975. We'd now now be seeing the impact of this adding up from both generations pushing the age of grandparents quite a bit older on average);
What the family relationship is like in both directions;
How much that person enjoys or more specifically doesn't enjoy kids in general;
How much free money they have to spend on the care;
Travel time to/from

Speaking from an Australian perspective about your specific example though, once a week seems like a lot of time, especially to make an exacting persistent commitment rather than organising it on a more ad-hoc basis.

We have an absolutely amazing relationship with my wife's parents. As an estimate, they probably end up helping out for at least part of a day in most fortnights and easily 1-2 full days every month, with some months easily double that.

I don't think I'd ask for a persistent once a week. That said, we also have government subsidised child-care and it's affordable for us so we don't really have a need for the regularly scheduled work-days.. our needs are the ad-hoc times that doesn't cover like appointments, school commitments, a random extra work day, school holidays, a date night/concert/party/dinner, etc.

I work full time (but from home, so I get to see my kids when home at least a little), my wife works 3 days (only 2 for our first child) so we also have been happy having them in daycare for only 2-3 days a week.. we are both very keen NOT to have them in daycare every weekday until school age and fortunately are in the situation to be able to do so. If we weren't, the desire for more regular grandparent care over daycare would probably be higher, purely for not having them in a rigid external environment 5 days a week.

But it would not surprise me if once a week or more was normal in some cultures or places. I know some definitely lean more heavily on the family unit than others, and some are also more likely to live in multi-generational houses which would likely lend to making that more likely - no one really has to go anywhere.

Some actual data from Australia showing 21-32% of grandparents provided care about once a week or more:
https://aifs.gov.au/research/research-snapshots/grandparents...

There is plenty more research and numbers on all of these things for many countries readily findable.