For us it was four days of no accidents at home, then she went to daycare in knickers on day 5. That (the TTing) was very kind of all of a sudden.

For shops, I think it's what you're comfortable with. Our first trips to the shops without a nappy, I took a spare pair & made sure the trip wasn't going to be too long & that there was a toilet nearby, and we went to the toilet before we got in the car (yeah both of us )
If you're comfortable with it, do it as soon as you want; I wasn't particular bothered by the idea of puddles. And like a PP said, I didn't want to confuse the issue by having knickers sometimes and nappies others (apart from bed time when she still has a nappy).