In the past few years, Apple has continued signing the old firmware for roughly 1-2 weeks after a major x.0 upgrade. But only Apple knows for sure how much longer they will keep signing iOS 9.
I personally think Apple stops signing too quickly, with all of our busy lives 1 week is too short. If anything 1 month should be more like it, it's not like iOS 9 won't work anymore, it's the most stable iOS thus far.