According to Official Guinness Records,
The longest duration balancing a chainsaw on the forehead is 5 min 1 sec, and was achieved by David Rush (USA) at Jack's Urban Meeting Place in Boise, Idaho, USA on 5 October 2017. David made his attempt to promote STEM education during the IVRC Virtual Reality Bash.
For a complete list of 2017 records, please visit 2017 Guinness Records in United States.