Firstly, what with the Tyranids supposedly being the eventual Doom of the Galaxy, the question is: How long could this take? Do they have a Warp Capacity of some kind? If not, eating their way across the Galaxy is going to take a hell of a long time. Even if they are Warp capable it could still take awhile.
So while resistance may be futile, it is conceivable that the end of all life in the Galaxy could be so far into the future could be so far off as to not matter; certainly not for the vast majority of people/xenos.
So, how long? Firstly without Warp Capacity. Secondly with.