What is long range? Anything well within the supersonic envelope of the cartridge in question, i.e., the bullet being fired. Extreme long-range could be thought of as anything at the outer edge of the envelope in terms of supersonic flight - and perhaps also into the transonic and sub-sonic zones.
Using a .308 with mid-range weight slugs, such as 168-grain SMK BTHPs, an 800 yard shot qualifies as long range, since under most conditions, that 168-grainer is going to enter the transonic zone somewhere around 850-900 yards, and start to become dynamically unstable. Firing the same load at 1000-1200 yards or more - as some military and tactical marksmen have done - would qualify as extreme long-range shooting, since the slug has already passed into (and through) the transonic zone and is flying subsonic when it hits the target. A tell-tale sign of an unstable slug is key-holing on the target paper... evidence that your projectile had started to yaw or sideslip.
How you define long-range and extreme long-range depends upon your gear, and of course, upon you. To someone experienced in ELR shooting, going out 1000 yards plus isn't that big of a deal; on the other hand, to someone inexperienced in long-range rifle, going out to 600 yards is a tall order.
My two cents, for what they're worth...