Some would question if this is even possible. I say it is, because what we think of as time is actually two seperate things. There's time as in the spacing between events, the kind measured in seconds and minutes, which is constant. and then there's the subjective experience of time, which can and quite often does vary, when something seems to take more or less time. This is the one you're varying, so even though it is only, for example, 30 minutes, it seems like 2 hours, which is the only relevant part because it's the subjective experiece that counts in dreams...like, I suppose, in waking life.