As mentioned, these bikes are good for a couple hundred thousand miles, easily, if well maintained. If the concern is stopping for fuel, it depends on as mentioned factors like single or 2 up riding, total weight with accessories like chrome or travel bags, your tire pressures, how much you twist the right wrist, surface you're riding on, riding into headwind, tailwind, if you have a windscreen or fairing, size of the tank depending on model being ridden, things like that. I have a windshield, and the smallest tank @ 4.49 gal. US. My light routinely comes on between 3.5-3.8 gallons, and is roughly 110-115 miles traveled. By then I need to stop anyway to 'get the kinks out', and hit the rest room. Now- for comparison, I was travelling a couple years ago with my buddy Renegade Ricky, and he had a 2004 S model, with a nearly 1 gal. bigger tank. We met and filled up at the same gas station, and headed out towards oHIo on I 70. After riding over a hundred miles or so, my light was coming on, and I pulled alongside to let him know I needed a stop soon. Within another mile or so, he started bucking and slowing down, as he was OUT of fuel. He'd been having an issue with no fuel light, and wasn't paying attention. His Harley fairing was acting like pushing a BIG shoebox down the road and consuming a lot more fuel to maintain highway speed. I'm also riding Darkside, or a car tire, so might have a tad more drag on the rear wheel than most. Just a few things to consider.
