2 things

(1) leveling to 50 and completing the storyline passing Ultima are NOT the same thing
(2) many people "claim" the 2-3 day thing after already having another 50 first. there is a big EXP boost that helps out a lot with this

I personally don't believe it is humanly possible to pass Ultima in 3 days, even with hardcore play sessions of like 18 hours per day.
Took me about 5-6 weeks my first time casually playing, watching all the cutscenes, reading the storyline, and playing around on other classes.

Personally I think it is possible to do in one week if you skip all cutscenes and rush through everything.