The short answer is more than you probably should. Compressing down feathers damages them over time but they can squeeze down magnificently.
Our testers have compressed a -40-degree bag into a 10L compression sack. That is not recommended but if you need space in your pack, you will do whatever you have to.
Packing down takes patience. Applying gentle pressure to remove air from between the down can squeeze it into tight spaces. Check with your manufacturer’s listed pack size. This is a good indication of how far to take it.
For example, one of our favorite sleeping bags, the Montbell Down Hugger, has a 3L listed pack size in the regular length. Compare that to the NEMO Forte at 12 L at a comparable temperature rating.
When you store your down sleeping bag, always put it in the large provided sack or bag about the size of a 55-gallon trash bag to prolong the life of the down and allow for full expansion while in storage.
Down backpacking sleeping bags compress down small, but pack some serious heat when you need them to; (photo/Honey McNaughton)