Building pull-up rig with 6x6's
General rule is 1/3 of the post needs to be in the ground. However, if the aim is to get a lot of height, consider going with an a-frame structure--essentially a swing set. 

Instead of burying anything in the ground, anchor it with earth screws (essentially a large screw used to anchor swing sets). 

So if you are using 12' lengths, and the base is 6' apart, you could still have about 11' of height without having to bury them at all (albeit you'll need extra material to make it an a-frame with two sides + cross bracing)