Depends on which temperature you ride your bike. I live in Delhi and ride everywhere from high temp summers to very cold winters. I would recommend you to flush after 20-30k kms ODO
You can use water as coolant flush.just don't use it as coolant.but using water can cause issues as it has dissolved minerals in it. Which can cause corrosion. Better to use distilled water, if that is not available use demineralised water. Avoid trap water as it contains chlorine.
