It depends of course on the hydrogen kit you are installing. Our Revoution HHO Kit takes approximately 3-4 hours, but we have a low maintenance system that has some extra components. Average install time is approx. 2 1/2 hours.
It also depends on what you are doing to address your vehicle's computer. If you are installing an EFIE and MAP Sensor Enhancer, which you should to see the gains with any fuel injected engine, it may take some time after initial installation to get them dialed in.
We always tell our customers, if you are not comfortable doing your install, please hire a mechanic.