Once Apple says "Out of stock", they could theoretically order another batch to be made, but that usually doesn't happen - it's generally an indication that the front end of the pipeline has closed, and we may still find them on store shelves and/or at third-party retailers until the supply dries up. There have been a few (some of the early Nike ones, like Violet Dust/Plum Fog and Blue Orbit/Gamma Blue) that seemed to go out of stock in mere minutes, while "most" of the seasonal colors seem to last until the next batch of seasonal colors are released (or nearly so).But to be sure I understand correctly-- once Apple says it's out of stock, Apple themselves will not be getting more-- correct? (at least online-- since you mention local stores may actually have some).
I suspect that if the current crop of seasonal ones runs low, it's more likely for Apple to order another batch - if they go with no "new/current" watchbands available for too long, that be bad for sales (don't want to tell customers "try back in 3 months at which point we'll have new new colors").