While there has been little secret that Apple will announce its headset next week (now we’re hearing that they’ve invited many virtual reality experts to the event), the question of when Apple will release the headset is much thornier. Of note is that we’ve not seen leaks from manufacturing. That usually happens once Apple hardware goes into production. That could mean that either Apple is not yet in production or they are making headsets in small, controlled batches.
Also of note is that nobody outside of Apple has seen the current state of the software. They could have the hardware locked down but are still waiting to finish the software. Historically, new product categories are the only place where Apple announces early. The iPhone, iPad, and Apple Watch were all announced months before product release. In effect, Apple is planting its flag in the product space.
I don’t think anyone outside of Apple knows when you’ll be able to buy a headset, and I wouldn’t be surprised if Apple isn’t a bit vague about that question when they announce with something like “later this year” or “early next year”. Regardless, if I had to bet a nickel, I’d say it is at least six months before you can buy one.