Congratulations on your twins! How exciting!
I've always been told that for the first 2 weeks, I was to wake up the baby to nurse every three hours, even through the night. It helps with milk productin, and a brand new baby needs the extra boost of nutrition at the very beginning. I would guess it's the same with formula as breastfeeding (not the milk production part, but the nutrition part)?
After the first two weeks, at night I only fed the baby if she woke up. But during the day I worked on establishing a feeding schedule (just like you might a napping schedule). For that, I did wake up the baby if she slept past her regular feeding time. Once the schedule was established, she rarely slept through a feeding time. The feeding schedule was a blessing! I always knew about when my baby was going to be hungry and could plan my day accordingly. It helps to regulate their little digestive systems too. If you want more info about a feeding schedule, let me know.