Doing vocal warmups is as vital before vocal performance as it is to an athlete before a game. How long should you do it?
... As long as it takes to be warmed up!
That means if you're singing regularly and in good physical and vocal shape, you should be great with about 10 minutes or even less of some wise vocal exercises (and yes, you should still do it). If however you are NOT in top shape or are waking up a little thick-corded, you could take as much as 30 to 45 minutes to be completely conditioned for singing.
Warm up smart... not hard. That means take it easy at first and don't push yourself til you've worked your cords a bit, then get more strenuous as your vocal cords tell you that they're ready. If you vocalize too hard at first you will experience a tighten up, not a warmup.
Also, please remember that when it comes to vocal exercise, form is everything!! If you do a great exercise with wrong form, it can actually hurt you instead of help.
OK... all ready? start with the bubble or trill..... hopefully you know the rest:)