How to calculate work/effort for PRP work?

[James Heinrich]

What is the formula for calculating GHz-days effort for assignments such as this:

PRP=1,10,5239909,-1,0,0,"9"

Mostly for my reference:

PRP=k,b,n,c[,how_far_factored,tests_saved][,known_factors]
The optional how_far_factored (in bits) and tests_saved values are used to determine if P-1 factoring prior to the PRP test would be beneficial.
The optional known_factors list is a quoted comma separated list of known factors of k*b^n+c.