How much time does a dream last?
as with many things, it varies a great deal. some dreams last just a few seconds, some dreams last much longer. though i’m not exactly well-educated in this area, from what i’ve read it can vary from person to person.
if you’re talking about lucid dreams then, from what i’ve read and personal experience (though not much of it), they tend to start off quite short, and, as you get used to the ability to control your dreams (and the shock/excitement from learning that you can), gradually get longer.
-satty
LaBerge said usually dreams are in real time. It can last to 30 minutes and so they equal those 30 minutes IRL.
However, some scenes in the dreams can be skipped so they sometimes seem lonegr.
Or there is something more behind it. But let’s leave those anomalies as they are not that well explored. 
Depends which dream. If you had a dream that lasted 15 minutes then it probably lasted 15 minutes. If you had a dream that lasted 2 minutes then it probably lasted 2 minutes. So it depends on the length of the dream 
Piculum is right although I would include rare instances where longer time seems to pass in the dream world than IRL.
This could be due to skipping as piculum mentioned, but I’m sure there are some that really do last longer than real life time. I have heard of people staying in LDs for hours, weeks, months, years. I wouldnt straight away believe the weeks/months/years but I do think it is possible with great practice and control to stay in a LD for hours by slowing down time.