Original Message

Yes that is legal. However unlikely. The forest service limits pilots to 8 hours in the seat. It is very common to to be on standby for 14 hours. As long as 8 hours flight time are not exceeded within that 14. Add another hour driving to and from the helicopter and 15-16 hour days are normal.
Now in other areas of part 91 there is no duty time limit. I have personally logged 15 hours in the seat doing power line work in one day. Often in the summer months 8 to 12 hours in the seat are common.