I think Objective-C will be around for a long time. It's certainly worth knowing if you want a job in Mac programming because chances are you'll be working with some old Objective-C code. As long as the old code still works, I don't imagine a lot of companies will throw it away and convert over to Swift, though eventually probably everyone will be writing Swift for new apps and updating parts of their code as they get updated.
If you're just a hobbyist, why not learn both? The more languages you learn, the easier it is to pick up new ones, and I'm sure learning Objective-C first will help you appreciate Swift and what Apple's trying to accomplish with it.
That said, Swift is obviously the future. Apple is going to be supporting it much more in the long term. It will get most of the attention at Apple HQ. If there's an OS 11 in the pipeline, it's possible Apple will switch to a brand new set of APIs and Objective-C will fall by the wayside, like C++ did after the OS X transition. In fact, I'd go so far as to say the introduction of Swift is probably the first big sign we've seen that big changes are coming to Mac development. A big company like Apple doesn't create and endorse a new programming language on a whim.
So, learn both, and learn as many other languages as you can, and you can be the old dev someday talking about how Objective-C used to be everybody's favorite thing. C, and C-based languages are probably not ever going to disappear, they just go in and out of style.