How long should I warm up before singing?

Five to fifteen minutes, depending on what follows it. Two minutes of breathing and gentle humming is enough before light practice; a rehearsal or a performance deserves longer, working from quiet humming through lip trills and sirens into full voice. What matters more than the length is the order: start below full volume, keep everything in the middle of your range at first, and never begin with the loudest or highest thing you plan to sing.

A guided warm-up, timed, with a sung example for every exercise