Not only months or even years, but thousands of years!
For example, see this:
Persistence of Avian Influenza Viruses in Various Artificially Frozen...
Background. This study investigates the viable persistence of avian influenza viruses (AIVs) in various types of artificially frozen environmental water and evaluates the feasibility of similar occurrence taking place in nature, and allowing for...
Or this post by @RonSewell
I agree that the idea of abiotic preservation cannot be just summarily dismissed. Not specifically influenza, but here is a paper surveying findings relevant to idea that microbes might survive long periods of freezing: Future threat from the past The remarkable finding of an ancient virus: Thirty-thousand-year-old distant relative of giant icosahedral DNA viruses with a pandoravirus morphology And some popular level articles: BBC - There are Diseases Hidden in Ice SciAm - As Earth Warms, the Diseases That May Lie within Permafrost Become a Bigger Worry Vox - Melting permafrost in the Arctic is unlocking diseases The permafrost pandemic: could the melting Arctic release a deadly disease?