NICE defines a very brief intervention in the following way:
A very brief intervention can take from 30 seconds to a couple of minutes. It is mainly about giving people information, or directing them where to go for further help. It may also include other activities such as raising awareness of risks, or providing encouragement and support for change. It follows an 'ask, advise, assist' structure.
Behaviour change: individual approaches (PH49).
Key NICE resources on very brief interventions are: