According to Official Guinness Records,
The longest time to dunk a biscuit is 1 min 12.3 sec and was achieved by Stephen 'Sizzlin' Steve' Kish (UK) in Glasgow, UK, on 27 November 2016. The record attempt took place in the BBC Scotland studios, after the filming of 'Officially Amazing' series 6.
For a complete list of 2016 records, please visit 2016 Guinness Records in United Kingdom.