You don’t actually make your dream itself longer by doing this. You don’t need to do any fancy space-time calculations like Valrys said.
Just turn a clock backwards, or say time dilate or time slow down. Since really all you do is make your perception of time slowwwww way down.
It’s would be great if IT 's possible and I think It’s possible
I tried this a while ago, but it didn’t work out to well. My goal was for the dream to go by slower, but actually, the entire dream went in slow motion. I remember seeing some girl across the street jump-roping, the rope would’ve taken 30 seconds to go in a complete circle, as opposed to the half second that it was at before. It wasn’t what I was aiming for, but its better than nothing. XD
I might try this again tonight, it’d be great to have dreams that last days
Oh, and I remembered, I actually tried it twice. Once was with a lucid pill, which did nothing. And one was by keeping aware and counting, and the result was what I posted above.
Slowing down time… I haven’t tried it before, but Its definitely on my to-do list now.
Well, in dreams we don’t really have a body, but our minds create one anyway since we expect it to be there when we look. It’s the same with gravity, it only exists in dreams because we expect it to. Therefore, I think that time goes by in dreams because we expect it to. I don’t think it should be an attempt of slowing down or speeding up time, but an attempt to imagine time as not existing in the dream.
Now, I’ve never had a long enough lucid dream to actually try this, so I might just be talking nonsense, but I think it could work.
just went through writerscube’s DJ suggested in here…
ld4all.com/forum/viewtopic.php?t … postorder=
a few weeks long LDs… 
my level of motivation has just skyrocketed by about 10 times…
https://community.ld4all.com/t/writerscubes-dreamworld-log-dream-diary-3/29215
August 24, 2008. That would be enough.
He also explains how he does it, later in the DJ. Have a good read 