It's all a pretending manner inside the dream... Like if you want somebody to appear in a room you'll have to "think" that this very person is waiting for you around the corner...

I had a dream that lasted 3 hours, though in real life it was just a couple of minutes...

You have to actually think that the dream lasts that long, not focus on the events at all, like if time was a problem. When you're actually aware that you might wake up at any moments, you'll actually see your vision fade in no time. By doing activities that doesn't stress you out, you'll actually go like time wasn't a problem. We tend to get excited when we lucid dreams so that's one major problem.

Oh and yeah, Inside the dream, slowling a clock or making the sun stop for exemple are good action to do to prevent short dreams. By doing so, it'll help you pretending that time isn't an issue...