6 weeks.
We started on breast, struggled with latch so switched to pumping & formula mix at a few days old.
Then he was struggling with trapped wind so we switched to breast only (with help of nipple shields for latch) at 3/4 weeks.
Then switched to fully formula feeding at about 6-8 weeks (can't remember exactly) because he was crying loads and it seemed he wasn't getting enough food breastfeeding only. We started mixing in formula and it gradually was 100% formula.
It was great because I could eat my dinner without having baby attached to me, OH could share the burden & baby wouldn't just nibble all night long. He'd eat and then sleep a few hours which was a godsend by this point.