In our airline, headsets must be used on the ground and in-flight below 18,000ft, and whenever deemed necessary by the captain. Also, both crew members must communitcate by the same means (i.e. headset or boom mike), so if the captain wants to keep his headset on all day then sorry, but so must you.
We also have the requirement for the remaining crewmember to go onto headset comms when the other pilot leaves his seat.