Reasonable help entirely depends on relationships, working patterns and incomes, in my mind.
My mother-in-law offered one day a week when I was on maternity leave with my first. We wouldn't have assumed or asked for anything. She worked part time, on a flexible pattern, usually 1-3 days a week. She's done 6 years of one day a week childcare now for us, but if she's away or has something else on then we find an alternative. I also put my younger one in nursery 3 days a week when I went back to a 3-day a week job second time around, until the older one went to school, so she didn't have both of them all day.
My MIL's childcare for us has caused some tension with my SIL who lives an 1.5-2hrs away and I think would have liked her mum doing more for her too. It's never straightforward! (We live less than a 10 min drive away from MIL)
I make sure to ask her a couple of times a year how it's working for her and make clear that if she wants to stop/change it that's fine. She's just retired and plans to be around to help more with number 3, which is amazing of her. But again I'm not going to be assuming anything - she's a busy person.
My FIL covers times when I'm stuck or have a late meeting - so maybe a couple of hours once or twice a month (and sometimes less), but doesn't do a regular slot. He's retired.
My parents aren't really in my life and wouldn't be appropriate childcare in any case.
I think the main thing about nursery, though, is often the cost. Some people can't make working work without some help with childcare and I know grandparents make a huge difference to that. Our local nursery (Z2 London) is about £90 a day for a 1 year old. Even with the help we've had there were periods where it was barely worth it financially for me to work but we saw it as a long term investment!