For how many days/weeks should I hand feed my 2 baby guinea pigs? They are with a sow with an older litter.?

We bought 2 female guinea pigs from our pet shop when they were 9 weeks old. To my surprise a few weeks later I went into the hutch one morning to discover 3 baby guinea pigs cuddled up with their mum and auntie. We then discovered the auntie (Ruby) was pregnant too. She gave birth 2 days ago but sadly died, along with 2 of the babies. We are now left with the 2 orphaned babies. We put them in with the other mum (Rose) and her 3 babies. One newborn still had the placenta attached and she ate that and cleaned it up straight away so we were hopeful. She did feed it a little but never for long and less enthusiastically than her own 3. The other one was allowed to cuddle up but could not even find a nipple to feed itself. I could only get it to feed by holding the mother up and latching it on myself. I was advised by a vet to top up their feeds myself which I have been doing every 2 hours. They feed well and have been nursing a little aswell and eating solids. How long should I?