I logged my hours from the very beginning. They include time on the couch reading the manual, watching videos and studying photos.
Everyone's timing will be different based on experience, selected options, and any help you can get.
Ultima estimates 350 hrs (400 hrs - 50 hrs for pre-fit).
I ended up around 440 hrs. This included extra options such as spraying underbody with Raptor, installing sound deadening and insulation, installing a stereo/speakers/subwoofers, installing a backup camera, and a few other items. Additionally, the 350 hr estimate is most likely for a naturally aspirated engine whereas I installed a supercharged LT4 with a dry sump. The S/C engines and dry sumps require a lot more plumbing.
Since "completing" the car, I have probably another 100 hours of tuning and tweaking. This includes tasks such as additional AIM programming, spark plug wires replacement (GMs are no good), catch can replacement (Ultimas is no good), front brake ducts tubing/mounting, redoing door edges, polish & ceramic coat car and a few other small tasks.