These babies may be tube fed with expressed breast milk. If you are expressing instead of breastfeeding, then you need to express as often as you'd expect your baby to feed. That way, your breast milk supply is stimulated. For a young baby, you should aim to express at least 6-8 times a day. It's up to you.
Some women express immediately after a breastfeed.
Some do it at the same time as their baby is feeding - with the pump at the breast the baby isn't on. Others do it between feeds. Find out what suits you best. At the very beginning, express for a few minutes on each side. After a few days, you will find you need to take about 30 to 40 minutes in total to get the maximum amount, perhaps stopping and starting, and switching sides.