Information on how service users are using your service, and the extent to which they use it.
To understand whether or not you effectively deliver the service to your intended users. Key questions include how often people come? For how long? How engaged are they?
Routinely. Like user data, you should be trying to collect this data on an ongoing basis—as and when people use the service.
How to collect?
The main method will be to rely on staff or volunteers to collect the data. To help them you will need to make data entry as easy as possible, encourage them to enter it routinely and ensure they are consistent in how they enter it.