1. Is there a limit on how far away an outlet can be from the GFCI that controls it?

2. If I want to put the outlet on a post how far above grade does the box have to be?

I would like to extend a buried line about 75 feet from a GFCI outlet and mount a weather tight box on a 4x4 post. Is the 75 feet allowable and how far above grade should the box be?