As far as you can place it in a vital spot.
My Creed carries 1k out to 800 or so. That is usually considered to be a "SAFE" energy level but there are so many other factors.
40 MPH wind, 800 is a long waaaaaay.
Dead calm on a bedded small deer and you just made a ranging shot and center punched a rock.....800 isn't that far.
I keep my AR-10 under 800 but I hunt out of a truck and always have a BIG 7mm or 30 cal AND a 338 5 ft away, so I never really need to use it past 800.
For my 6.5cm, pushing a 143eldx at 2785fps, the longest shot on deer that I'm comfortable with would be about 650-700 yards. I know it will go further, but that's what I'm comfortable with...I have other rifles if I need to go farther. Just keep in mind, the 6.5cm was never intended to be an ultra-long range caliber.
I would consider 700 yards a good max range for the 6.5CM(or 260R) on a mature sized whitetail or mule deer. Using a 6.5 with a good, high BC 140, you can count on expansion and killing power(1000FP), if the bullet hits the animal in the vitals at a miinmum of 1800FPS. IMO.
"My" unwritten rule is 1000 FT-LBS of energy at point of impact and of course shot placement is still the key.
For instance, if I use a factory ammo like 140 A-Max (2710 FPS, 2283 FT-LBS, .585 G1) and use Hornady's ballistic calculator, at 650 yards it's 1807 FPS and 1015 FT-LBS.