When air pressure is reduced in the air stream emanating from the lungs, this results in diminished pressure and tension of the vocal folds (used to be called "vocal cords"). Three simple ways to do this are: warming up with lip rolls (lip trills) OR on tongue trills as in a "rolled R, like in Spanish or on a phonated 'Z' and done on: tetrachord plus one-up and down, on arpeggios or even on octave glissandos-ascending and descending. These also can be reversed pitch-wise. Now the good news. There are vocal coaches who teach this and also charge as much as $500 per hour. I, myself, paid $175 an hour for this and to learn much more, a few decades back. Over 200 Grammy winning singers and singers from all styles and genres, including Broadway warm up like this. Why? Because it works and it causes no strain (if you do it correctly) and it can help ensure a career that lasts and a voice that is freed up, relaxed, and endurance is better than NOT warming up. It is better than the old bullshit things done in most choruses led by pitifully inept people and also by the vocal coaches who should not even be in the business, spouting all the worthless clichés, most of which reveal absolute ignorance of anatomy, physics, and acoustics. For instance, if you think you "sing from your diaphragm" you do NOT know how it works. Talk to a pulmonary physician if you want some expert opinion on that and if you think you sing from your diaphragm after that conversation, you might want to see a physician who works with people with delusions. The diaphragm has no proprioceptive nerves in it, so you cannot even feel it. Do NOT blame me because I did not make or design your diaphragm. It is also the "INHALE muscle" and the expiration of air is done by the elasticity of the lungs and thorax. It is NOT my fault. If the expiration of air is forced, it is done by the internal intercostals, the triangularis sterni, and the rectus abdominus (abdominal muscles). WHY would I mention this? Because if you distract yourself with typical misinformation, not only will you never improve, but you also will have your attention in places of fantasy and your musicianship and your sound and your performance will suffer greatly. So... I did not design or make the human body but I have taken the time to study anatomy and to work with some physicians (5 of whom were students of mine) to verify that my information is correct. I am not the source, so don't attack me.

How long should a person warm up? Until the person is warmed up. Some days it could be 5 minutes. Some days it could be longer. People are biological but not always logical. Life is not the same as inanimate objects, possessing predictable quantifiable control or results like structural steel and operate within the confines of the laws of physics although those do also apply to peoples' actions such as: inertia, momentum, acceleration and other principles. If a person warms up for 20 minutes or more and is having any difficulty singing, it is not the warm up which is the issue. It is lack of correct training, done correctly and a failure to comply with the participation with the knowledge, which we call practice. This brings us back to a person at rest tends to remain at rest unless acted upon by an outer (or inner) force...inertia