I just did Monstaliner to my rig. Original white to White liner (front was repainted white at one point and the shoddy "clear" was pealing) back hatch was severely rusted along with doors and rockers and rear quarters. Took from November to now (as I almost have it all back together) to do the whole thing taking off all doors, hood, hatch lights, windows, door handles. Basically anything that could come off I took off.
Granted I have a WI truck that's Dented / Dinged and Rusted with 300K and doing this might of prolonged the body's eventual death for hopefully another 1-20 years.
Honestly I'd never do it again. Its a pain in the ass to do it right and I'd say I got it about 75% right taking 6 months to do it and basically stripping the truck to a shell. Sanding (do the DA sander - 120 Grit), taping, spraying, rolling. No matter what you do rolling or spraying, the liner gets everywhere - Your shop floor, Parts - you didn't want speckled, yourself, Frame, Sliders, bumper ect.... Plan if you are changing colors to at least have to do 2 coats to do it right.
Don't get me wrong it looks good/ok and I already backed into a tree and the liner didn't even get phased - the main reason I did it was to hopefully ward off some of the rust and cover up existing dents, huge scratches in the paint.
If you have a rig you plan on never getting rid of or wheeling the crap out of, then I would say do it.
If its your DD and you are someday hoping to sell it for big dollars, I wouldn't do it as you are narrowing your list of "potential buyers" down substantially.