Cola's info is pretty much on target. The majority of curing takes place in the first week. That pad can handle a lot more weight than a hot tub. I just poured a pad for a trash dumpster and put it to use in a week and that involves a trash truck backing into it and lifting and dropping. Even a house foundation only sits a week before the framers start. 28 days is a long time to wait, IMO.
That's a good point about the house frame. I used to live in a development that was adding new houses on a daily basis, and I never saw them wait a month to start framing. And if I read Doc's graph correctly, it looks like the concrete is almost as strong as it's going to get after about two weeks.
