As with any gear, sleeping bags need to be taken care of. With proper care, as Hikermor states, they can last a long time. I have found that synthetics can take more abuse than down. As you all know (I hope), storing the bags tightly compressed is a bad thing.

Tumbling dry sleeping bags can be a good thing on low or no heat. It helps loft the insulation back up whether it is down or synthetic. There are many new non-detergent washing powders/liquids, they help prevent a build up of residues which limits the insulating capabilities. Down insulation tends to clump after washing and needs the tumbling to separate the feathers.

At least this is my experience. I do have some old bags that still work well, but not as old as Hikermor's.