This will really depend on how strong of a skier you already are and how prepared you are going into it. I taught myself to manage bumps in one day back when I was a sophomore in high school. I wasn't skiing them beautifully, but I was getting through them in a controlled zipperline-esque fashion (straight-ish skis back then). I was already a decent skier when I decided that I was going to do it... but that was the key - I decided that I was going to learn to ski bumps come hell or high water. I still hate the things and will avoid them at all costs, but I can manage them if I am put in a situation that calls for skiing them.
So... I definitely think you could learn them in one day if you do some research ahead of time on what the best way to approach them is, and get a good instructor. Beyond that, you need mileage. When you go out to learn bumps that doesnt mean take a bump run and then a groomer run... It means you ski bumps, all day, up and down, without stopping to take an "easy run." Ski the same trail and same line until you get it right, then move to the next one to up the challenge. Eventually the tactics and skills that you learn on "your line" will become natural and you will find youself with the ability to apply them to many lines on different trails.
I always joke with people I ski with because I will ski the same trail over and over again... their jest is "we must be skiing this run until we get it right.
:" There is some truth to it, and it works if your interest is to become a better skier. The reason being is that by the end of the day you will have memorized the terrain that you are skiing and that alone will be a huge confidence booster as you tackle unfamiliar technique/terrain. by the end of the day a trail with once foreign terrain will not seem so foreign.
The best advice I can give regarding bumps is to stay loose and flex/extend. This will help your skis maintain contact with the snow and help you maintain speed control. After seeing my bump skiing I may not be the best person to take bump skiing advice from... but it's better than nothing I guess.