I think they’re “ready” by around 2.5/3 for half days (9:00-12:30 / or 12:30-3:30). And by 3/4, for full days (9:00-3:30).
Of course, some people need a lot more than that because of work! And the children are absolutely fine and having fun.
But if you have the option, and you’re enjoying being at home, I’d go for half days from about 2.5/3 until they start school.
School nurseries do three different types of sessions - morning or afternoon or all day.
For me, personally, I picked the afternoons as there were more activities on in the morning. So I could pop out, do a thing, drop at nursery, have a rest at home and then pickup. It worked great! But you can’t pick and choose days at school nursery; you have to take all week. That was the downside (I did it for one of mine the year before they started school but not the other because I found the all week thing a bit restricting - my other child went to a private nursery for two days a week instead, 9-5 which was quite a long day but they absolutely loved it there! Me too, I wanted to stay!).
If you’re going to do school nursery, I’d also be inclined to choose the nursery that your child will realistically be going to school at.
Also - if you’re running out of friends with children who are at home, nursery is a great way for both you and your little one to make new friends. My older child is 11 and his best friend is someone from nursery, that he didn’t go to school with! (But I was happy to facilitate that friendship because I really liked the parent, it would have drifted otherwise!)