usually, the bike comes with a manual and teaches you how to put a whole bike together, so i adjust mine myself (i learned myself through trial and error how to tune up bikes) anyway, it takes me 5-20 MINUTES depending on the stuff I'm tuning. it also depends on the size of your bike shop. if they have many other bikes to work on then yea, it might take 20 hours. But 20 hours all together spent on one bike wouldn't be worth it. I'd say buy a new bike if some kid asked me to put 20 hours in a bike.