Some people are really overthinking this one, while others are looking at it too simplistically. You can't just calculate based off of muzzle velocity, because a bullet is slowing constantly after leaving the rifle. But, you can still use a ballistic calculator for the load you are talking about, and you'll have the answer instantaneously. For example, a 140 grain Berger Hybrid Match bullet from my .260 Remington will reach 1 mile in 3.05 seconds. That number is also controlled by the atmospherics at play at the time that the shot was taken. My numbers are based on a density altitude of 7,000 feet, since I live well above sea level.