Q: When will iWeb no longer be usable?

Why not make an appointment with a Genius at the Apple Store? Perhaps he can see into the future.

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What won't work?  The iWeb application or your published website?

If the former probably until the next full version of the system, i.e. 10.11. If the latter  I'll guess around 20 years as long as you continue to pay for online hosting.

However, if I were you I'd change applications and redo you website so you'll be able to keep it up to date and have the features that newer applications offer.  One application that many have migrated to is Everweb because it is very much like iWeb in it's user interface and drag and drop features. It has a 30 day demo you can try to see if you like it.  I definitely wouldn't start out now using iWeb. 

OT,

I think the OP was just asking generally, how long the iWeb application would continue working on a Mac.

Also, here is the link to EverWeb.

Thanks for the link.  Forgot to add it to my EverWeb reference.