Greetings!
I am a fan of fixed-power scopes, typically mounting a 4X to anything .22 or smaller caliber, and 6X or 10X for long range rifles. My current long range performer, a Ruger M77 MKII in .243 Win, is topped with a 10X fixed power scope which I find to be more than adequate for anything up to 500 yds. I think it would probably be just fine for anything up to 800+ yards, but I don't currently have access to a shooting range of that distance like I once did.
I did quite a bit of shooting at 700 yard range with a .308 Win. rifle 8-10 years ago which had a 3.5-10X scope that I kept at 10X all the time, and 10X was very adequate for 700 yard shots with that rifle. Like the previous posting says, whatever you are comfortable with... I have taken dozens of shots through open sights with a .30-06 at ranges up to 300 yards and haven't felt "undermagnified"... although I generally prefer at least 6X for high-power rifles.
Best wishes,
Bawko