Do you think it is useful to change the chain without sprockets ?
It depends on your transmission and materials from which it is built.
I don't want to go into when I used derailleur systems, let's just say I was continually p*ss*d off at the waste and inconvenience. But those transmissions accustomed me to not expecting more than a thousand miles from an entire transmission system. I replaced the sprocket set with all the other gearwheels because they all got wrecked at the same rate as the chain.
When I went over to hub gears, I used the beautiful but soft alloy Nexus crank, sprocket and Shimano chains, and they wore out together at such a rate they never managed much better than a thousand miles.
This couldn't be right. Most of the cyclists I knew were unsmiling idiots, who believed all the nonsense on television from the climate catastrophists, and got angry when anyone didn't instantly believe the street corner gossip they retailed as gospel bicycling lore, but a few were thoughtful people, and very helpful when I made a determined effort to educate myself about bicycles.
As a byproduct of them directing me to bicycle makers who knew what they were doing, I made a major discovery. I was unsympathetic to the mushy aesthetics of the crank Utopia-velo fitted as the default option. I wanted something more classic, and while I decided what I wanted and sourced it, I told the dealer preparing the bike for delivery to me to fit a cheap crankset and give me a credit for the expensive but ugly default crank. He fitted a forged steel crank and chainring from an Indian manufacturer I'd never heard of, Amar, and this, together with KMC Z7 chain and high-quality Rohloff sprocket, was an eye-opener.
I was now in position where I got three to four times the mileage on a single chain than I had before. I want to stress that all of these transmissions operated inside full chain covers, first the Dutch plastic kind, several, then Utopia's own Country, then the Hebie Chainglider, and that my maintenance was state of the art, from white liquid wax every hundred miles or so to my experiments with running the chain solely on its factory lube inside a Chainglider for its entire life.
When I finally found cranks I liked, I discovered that the high-quality Thorn chainring I'd decided on was too wide for the Chainglider, and substituted a Surly stainless steel chainring. I now run X8 KMC chains, arguing that if the fixie Z7 was good, the derailleur X8 with its extra flex would give me an extra edge. (I have no idea whether it has worked; I don't keep the sort of detailed records Dan and Martin delight in. But the X8 is available everywhere and often on sale, and usually cheaper than the slower-selling Z7.)
Then I fitted a Bafang centre motor, which requires a dished chainring, and bought a selection of their available tooth counts that also match the Chainglider's availability for the Rohloff -- and rediscovered the virtues of steel.
I last inspected the sprocket on the Rohloff at around 11K kilometers, and there was no wear. The Amar steel chainring was on the bike for about 3000km, and while it could have done with a wire brush on the back and some Rustoleum, the blacking on the teeth wasn't even disturbed. I'm sure no one will be surprised to hear the Surly stainless steel chainring wasn't marked in the few thousand kilometers it was on the bike. The current Bafang dished chainring isn't showing any wear in about 4.5K km.
The common element is steel chainrings and a high quality Rohloff sprocket. I have throughout taken the view that the chain is the cheapest replaceable service component on the bike, and thrown off the chains at around 0.5 percent wear, in part because I wanted to start experiments with a new chain.
So my answer, based on experience, is, yes, on your Rohloff bike with the Rohloff sprocket, you can change the chain without changing the sprocket.
About your second question, after how many miles should you change the chain, that's going to be different for each of us.
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I also think that there is a case to be made, especially with the Rohloff sprocket, to ride the lot until the sprocket (or more likely the chainring, if it is aluminium alloy) starts skipping, that is, let them all wear together because that way they will fit together until they don't, at which point you flip the sprocket and the chainring and fit a new chain.
However, it may also be that with such extremely worn components, the outcome may be perverse in that it will lead to one component with some wear still left on it but being too worn to accept a new chain. That outcome can perhaps be postponed by running several chains in parallel, say for a thousand miles each, so that there is always only a small difference in wear between the chain and the sprocket, ditto for the chain and the chainring. Considering the amount of work involved, at this point I lose interest; I'm happy with my near-zero maintenance bike and consider chains a near-irrelevant expense; I'd probably have a different outlook if I were a major tourer. It would make a worthwhile experiment for someone with enough years left to let the components do their own thing to the end.
