For global reporting, countries are responsible for collecting and sharing indicator data and metadata, which will be compiled and validated by the indicator custodian agencies. The custodian agencies will then submit the validated data to the United Nations Statistics Division (UNSD), to inform follow-up and review at the Highlevel Political Forum on Sustainable Development.
Although UNSD will produce its Sustainable Development Goals Report on an annual basis, the frequency of country reporting on each indicator may be different and depending on the individual indicator. For some indicators it may make sense to report every or every second year, for others it may be sufficient with less frequent reporting.
For SDG 6, the custodian agencies are sending requests for data to countries on a regular basis. For an overview of the 2016-2017 requests for data, and the suggested frequencies for reporting, see this page.
Overview 2016-2017 data collection process and timeline