Forever if you take care of them. My Dad's McIntosh equipment is still being used and he had it before my brother was born.
So - that was over 45 YEARS ago.
Speakers may eventually wear out, but a nice cabinet will last forever and you may need to replace drivers 10 years or so down the road.
Lots of good subwoofers out there, but perhaps worthy of its own subject. I personally scour eBay and audiogon.com for deals. Picked up my HGS-18 Velodyne sub for about 1200 bucks. Used can be simply awesome on both your ears and your wallet.
