Edward, too many variables to give a specific answer. What do you mean by long range? 1000 yds? 2 Miles? Could be anywhere from 22" -38".
What you need for barrel length is very much dependent on what you want from the rifle, as well as caliber, twist, bullet weight and intended distance you wish to shoot. You'd get better information for your specific intended use by googling your question with a few more details.
Stay safe & have a great day!

Edward, as others have said alot of variables but mostly depends on the caliber. For 5.56/223 alot of long range competition use 18-20 inches. Now 6.5 creedmoor 24-26 inches, 308 I'd go 24-28 inches. Another thing to watch is twist ratio. For longer range you'd want to use heavier projectiles so you'd want a quicker twist ratio to help stabilize heavier grained ammo. If you post again with the caliber, average distance you plan to shoot, if you have specific ammo in mind can give a more detailed answer. Hope this helps some.

Hi Edward, this will depend on the caliver, bullet coefficient, velocity, barrel length and burn rate of the powder used. For any given caliber, you will want the highest bullet coefficient, powder burn rate optimized for the given barrel length used and a qualified shooter which is as important as the other factors. Shorter barrels can be just as accurate but yield lower velocities for shorter range potential. If you could let us know your firearm, barrel length and ammunition we could give more specific info. Cheers and stay safe!