When my DC was in reception, there were a couple of SAHMs who were very clear that this was "their" time now the kids were at school and they finally had a few hours to themselves in the middle of the day. They tended to use to to walk the dog, go to the gym, clean the house, cook dinner in advance and just take a bit of time for themselves, having had the relentlessness of small children at home for a number of years beforehand. And it's not like it was the whole day - school hours are 8.45 to 3.30 and then they're into the whole round of school pick-ups, ferrying kids to activities, supervising homework, reading, doing dinner, bath and bed. Usually by themselves with their partners home late. Weekends are spent on family days out and taking kids to activities.
Tbh, it doesn't sound like a very lazy life to me, I'd probably describe it as "leisurely" and when you add in things like sick kids, helping with school trips and being on the PTA and doing home improvements (which lots of people constantly seem to be doing), I can see how they can fill their time. I work part-time (admittedly with a toddler as well as school child) and DH and I are discussing adding in an extra nursery half-day when I'm not working to give a bit more flex to get things done.
Of the mums I can think of who were in this position, two now have jobs, one is retraining, one is doing a university course and one has had a new baby. One is still a SAHP with one child but she has significant caring responsibilities for elderly parents, I think, and even if she didn't, no one would bat an eyelid - it's not our business how other families organise themselves.