I suppose one could debate the term immediately but, yes, some sort of response would happen if the "back off NOW" message was ignored.
I don't mean to open a debate on semantics, just emphasizing a reasonably immediate military reaction.
May I ask, why you believe NATO would react if the "back off NOW" message was tactfully ignored or subverted by Russia, much as they are doing now? Honestly curious.
Problem is, NATO couldn't afford to ignore the Russians aggression without losing some serious face or credibility as an organization, if it survived at all. I mean seriously, how long would NATO last if they said, "An attack against one is an attack against all," then changed it when it wasn't "expedient" for them? Why would you place conditions on that?
One of the biggest reasons that kept the Russians from rolling across the Fulda Gap for nearly 40 years was the fact they DID recognize that they would be taking on ALL of Western Europe, and when it all came down, would they have REALLY been able to rely on their Warsaw Pact allies? Case in point: the Poles are far better allies to us then they ever were for the Soviets. Even if we (the US) do carry most of the weight, they are quite aware of our capabilities and technology. Iraq has shown SOME of that, but not nearly all.
Bottom line, Putin might be trying to restore Russia to her old Soviet glory (whatever that might be) but even he's smart enough to realize that he can't go toe-to-toe in a protracted war with the West--he doesn't have the money, he doesn't have the manpower, and he doesn't have the material. Too much of his military sat around rusting for the better part of a decade. Sure, he might have some Backfires and a few new Su-31's, but exactly how long would those last when faced with the combined military of nearly 30 nations?