If it's a Strap, there's a good chance the answer is never. Every Strap we make is limited edition, so once it's sold out, it's gone for good. However, there are rare occasions where a Strap sells out before we can put the finishing touches on all of the Straps that were made, so there is a very small chance you could get lucky there.
For Imperials, we'll be restocking those until further notice. If you see a notice on the product that shows it will not be restocked, then that design is gone for good.