Down is actually a pretty durable insulation, and if it is properly cared for will last much longer than any synthetic insulation. Some people use the same down sleeping bag for decades, but there are a lot of variables at play, and maintaining a down sleeping bag is a bit of an art form.
When you buy a used down sleeping bag you will want to know:
- How was it used? (Long trips, short trips, with or without a sleeping bag liner, etc.)
- How was it stored? (Down should never be stored in a compressed state and should be kept away from moisture as much as possible, as discussed here in a previous question.)
- How was it taken care of? (For example, tears in the shell fabric may mean the bag has lost some of its fill. Discoloration could indicate UV exposure, or mold and mildew.)
Here's a brief article that outlines some things to take into consideration when deciding whether an older/vintage sleeping bag is still usable.
If I were to buy a used down bag, I would definitely want to know whether the sleeping bag has been laundered regularly (a good thing--to reduce the build-up of dirt and body oils), and what kind of cleaner was used. Detergents destroy down's loft by coating the feathers and causing them to stick together/compress. Dry cleaning is equally bad, because it strips down of its natural oils, also reducing its loft and insulative power.
As for an example, a friend of mine who has had a 20-degree Fahrenheit bag for about five years now will only use the same bag down to ~32 degrees Fahrenheit. This bag has seen extensive use on long, multi-week and multi-month backpacking trips in humid climates. The bag's owner says that using down-specific soap such as Nikwax revives loft temporarily but the bag will never return to a like-new state.
Another consideration to take into account with the military surplus type bags: part of what influences the price of sleeping bags is the fill power of the down (700, 800, 850, etc.). Higher fill power down provides more loft, and therefore more warmth per weight. It is also more costly. Army sleeping bags, where cost savings are generally more important than weight savings, are most likely using a relatively low fill power down. Also, if down appeals to you for its outstanding warmth-to-weight ratio, remember that if the sleeping bag uses a very heavy shell material (as is often the case with older gear), it cancels out some of the weight savings of using down fill.