This information will be collected by and available to those who need it - the airline involved and Eurocontrol CFMU, for example.
An airline will typically monitor every flight duration for each individual flight number. This will enable them to create a frequency distribution which will be pretty much a normal bell-shaped curve. They can then apply this to their own in-house policy to determine scheduled flight times. For example, they may aim to publish a scheduled block time which has been achieved on 85% of occasions in the latest schedule period. In this way block times may be adjusted over time.