As the PP said the suck reflex comes in at around 34-36 weeks gestation. It took DS1 (30+1wk) 5wks 4days to finally suck feed. He had a very weak suck and also some other issues and was unable to attach properly without the use of a nipple shield.
DS2 (34+3) went from IV/NGT combo to suck feeding within 4 days, he was fully breastfed by day 6-7.

Each bub is different and some take longer the others. Arm yourself with a NICU LC early on, they are a wealth of knowledge and help.

All the best x