How long does it take to lose 20 pounds?

10-20 weeks assuming you don't include loss of water weight and eat and exercise appropriately.
Work out an hour a day=50 min INTENSE cardio 10-15 min weight/abs is enough.
Count calories=if your weight is steady right now cut out at least 500 calories a day, if you are gaining weight currently cut more out, don't go much below 1500 calories a day though.

True diets are a lifestyle change so make sure you commit yourself to eat healthy from the day you start. As for time wise, I'd say about one to two months depending on how exercise you do and which type of exercise. Weight training + aerobic is ideal.

Generally, it takes around 3 months. It all depends on how much you put into it, though. If you're planning on dieting, make sure you don't starve yourself. Instead of cutting food out altogether, replace the junk food with calorically-efficient fruits and veggies. It may seem cliched, but for a good reason: it works. You'll honestly drop weight faster and look better by eating healthy than by not eating at all. (I'm sorry if you already know all this, I've just gone through some difficult experiences with friends who quit eating, and I don't want anyone else to fall into that trap. )

In a healthy way it depends maybe a few months! The non healthy way 2 and half weeks for me anyway but don't do that because you just end up fatter than you were when you started so do the healthy way