Generally speaking, once a bullet is gyroscopically stabilized, it will remain so, for practical purposes. It will hit the ground or the target before it destabilizes. The resistance to it's forward movement is much greater than the resistance to it's spin. In other words, the spin doesn't slow down much and the spin is what makes it's stable, not the velocity.
I think tests showed bullets slow their rpm rate down about 10 to 15 percent over 1000 yards; much less than velocity drop off.
As long as the bullet remains supersonic, that's true When it goes transssonic, they become destabilized and move off the trailing trajectory axis and start to keyhole in targets. Example; M852 7.62 match ammo with Sierra 168's leaving the muzzle at about 2600 fps (2550 fps at 26 yards as spec'd) started doing this at about 800 yards in cooler tempertures.