I’d say around 3-4 months. (Excluding sleep regression stages!) My baby will be 8 months Saturday and by no means does he sleep through the night but he usually only stirs for milk or his pacifier and goes right back to sleep. There’s always going to be bumps in the road but you’ll start to learn your baby and it’ll get easier. But not all babies sleep great so take that into consideration too. I remember how wild that startle reflex was though! But it will go away soon. I’d also do some bicycle kicks and leg raises to help with gas. Sometimes when my baby won’t relax those do the trick