We started around 4 weeks. I think that when this question was asked before, people said they started as early as two or three weeks. Your LO probably won't do much other than lie there for the first month or two, but it's good to get them used to it.
Sorry, I should have looked to see if this had been posted before. I was being lazy. Good to know. DS was 3 1/2 weeks early, so I'm just now starting to feel good about doing the normal things moms with newborns do.
I have been putting him on his belly since almost day 1. Sometimes when he is super fussy, laying on his belly with me rubbing his back calms him down.
Re: S/O Tummy time