How long do the Sennheiser CX 300 In-Ear Headphones last?

A LONG time, if you keep them in good condition. In-Ear heaphones can last FOREVER, as long as: 1) You don't wind them up- this can break the wires inside in the long run. 2) You don't get them tangled- too much tangling, and again, the wires go 'SNAP'! 3) You keep them clean, and keep them from hooking onto things. Overall- they should last a long time, if not FOREVER.