Dependant on conditons such as how hot the water you use and how long the fan runs make a big difference. Under severe conditions the speakers would be hard to listen to after 9 months. At a more optimum condition I estimate the speakers to last 7 years and 3 years for average with a paper cone speaker.

I'm going to say that the more you use the speakers under this kind of condition the longer they will last. If you take the grills of the speakers and the woofer's are a poly cone then you very minimal chance of a problem. You can get outdoor speakers but I dont know if you want a moss green rock sitting beside the hamper or not. You can alos try and treat the cone on the speakers if your really into a project. Remember you are going to have to remove the driver and treat both sides of the cone so it is sealed from the elements at large.

Cone treating with sealer like shellac and other water proofers can change the tone and specs of the driver small amounts. Probubly not an issue for bathroom speakers.

Bill