The initial rotation rate of the bullet depends entirely on the muzzle velocity and the twist rate of the barrel. For instance, if you have a rifle with a 1-turn-in-12" twist, and the muzzle velocity is 3000fps, then the rotation rate is 3000 R/sec, or 180,000 RPM. If the twist rate was 1-in-10", at the same 3,000fps MV, the rotational rate would be 3600 R/sec, or 216,000 RPM. After leaving the muzzle the velocity and spin rate become independant, and I've never seen any data regarding the rate at which the rotational speed slows over time after leaving the muzzle. I would guess that the rotation speed sustains better than velocity because of the small amount of drag the bullet would encounter that would resist the spin, but that's purely speculation.

Mike