Let's answer the OP?
There is no hard and fast rule, stephen, except for mindless automatons who switch them on and off at 10,000' +/- 1 ft because the book says so.
Lights are there to be used by the crew. They can be used to determine if you are in cloud at night (for icing reasons), for aiding anti-collision, for alerting aircraft on an airfield that you are about to either take-off or land, trying to alert birds to your presence, as a 'reminder' to crews of a particular ATC clearance and probably a few other reasons, not forgetting actually 'landing'.
PS Did you mean the HP Hermes?