They should have as much as they want as often as they want. At 7 weeks, I would say that was probably around 60-90ml every 3-4 hours, maybe 8 feeds a day, but that was partly breast milk and partly formula. She wasn't a very big eater. But really as much as they want and whenever they seem hungry. If your LO is draining every single bottle, I'd offer more until you get to a point where he/she is leaving a little and being satiated after a feed.