Actually anytime:) I probably started when baby was about 2 weeks old. I placed baby on my chest for a couple of seconds each time..then gradually increased the duration. As baby got older, I would place her on a firm mattress and lie down beside her so that she can see my face and not be scared. As she got stronger, I would play with her during tummy time and give her toys to look at and try to reach out to.
I have started after a month