How far must mechanical protection be provided when underground service conductors are carried up a pole?

Who's pole?

If this is the utility pole then they have their own spec. Most call for ten feet up and then they take over from there.

Up a utilty pole?

The entire height of the conductors, up to a comfortable location that the conduit can end and the conductors loop out to connect to the aerial lines.

You should need a weather cap at the end of conduit.

A limit of height may be required by the utility company, depending on the pole and any utilities that may be on it.

Consult with your utilities.