My workouts take about 25-40 minutes usually. The problem is, this isn't "just do 1 set". I carry those sets to failure and beyond and they are difficult. I may spend 5-7 minutes recovering from a set of a certain exercise like deadlifts. I'm not talking about hanging out at the water cooler, I'm talking about gasping for air, not even thinking about talking, type of recovery between sets. As you can see, I typically do 2 bodyparts/workout. This means usually 2-3 warm-up sets and about 4-6 working sets of all-out effort.
Tonight I train legs and the timing will probably work out like this. Warm-up with a set of squats. Maybe takes about 1 minute, take about 30 second break and then do a warm-up set on leg extensions - warm-ups will be 2-3 minutes then. Then, about 1 minute break and I will do an all-out triple drop set on leg extensions. This may take me 90 seconds or 2 minutes. I will need a good 2 minute break after that. Then I will do a set of squats to failure. Another minute of exercise and probably a good 2-3 minutes rest will be needed. Then I will do a set of leg presses with 2 drops. Another minute and then I will probably need at least 5 minutes to rest. Now I am at 15-20 minutes and I will finish with a set of straight-legged deadlifts - so now I'm going to be around 22-25 minutes. Any delays setting things up and moving weights around may bring this to 30 minutes. I could probably, and probably should, push the rest periods a little harder to get my time down a little more....but it works for me. Oh, and then I will barely be able to climb the stairs to get out of the basement.
Of course, chest and biceps will require shorter rest periods, so that might be slightly shorter.
If I'm not pushing the drop sets, forced reps, etc. and just going straight in, I've finished a workout in 15 minutes and I have no problem with that at all.
I think that you may be missing some of the point in just focusing on "it may only take X amount of time". It is like when people say, "Just 1 set to failure? That will never work." The real power behind the idea isn't that 1 set is all you need - because it may be more. The real power is that 1 set MAY be all you need and you should be searching for that specific amount that is necessary to stimulate muscle growth/adaptation (which is your goal).
Can you design workouts to be under 20 minutes? Easily. I have had times where, instead of doing a drop set on leg extensions, I superset them with squats, and that will cut some time out. I've also experimented with doing just leg extensions, squats and straight legged deadlifts on one workout and then replacing squats with leg presses on a second workout (More of a Mentzer approach) and those workouts were 15 minutes each or so.