One should get iPhone X battery replaced when the battery health falls below 80%. Anything above 80% is considered excellent. If used well, the new battery should last minimum for 2 years.
Coming to price, any nearest Apple iPhone Service Center will charge between ₹6500 – ₹8000 for iPhone X Battery Replacement.
In the local market, you can get it done for around ₹1500 but the battery will not be original, and will not long last.
If you don’t have any Apple service center near you or their charges are out of your budget, try getting the battery replacement done from some online repair service provider.
iFyx is doing the iPhone X battery replacement for ₹4500/- all-inclusive price and offering 90 days warranty. They have service across India.
Whichever service provider you choose, make sure new battery shows battery health of 100%. Also get proper bill, and warranty from the service provider so that in case the battery is found to have some defect, you are able to claim warranty.