Assuming you aren't traveling at relativistic velocities or chillin by the event horizon of a black hole, time passes at a constant rate. As living organisms, we perceive the passage of time as it is interpreted by our brain. Consciousness is the end result of our sensory input interacting with our brain. Even vision doesn't have a direct link to our consciousness - it passes through many 'filters' and specific processes of analysis before it is brought to our attention. So it is impossible to be have a true objective perception of reality. Anything that messes with one of the hundreds of perceptual filters and analyses will distort this consciousness. It would be like changing the effects settings on a digital camera.
Marijuana affects parts of the brain dealing with memory and memory integration. One could say that memory integration is a large part of experiencing the passage of time (this happened, then this happened, ect). If you don't remember anything from a dream, you wake up with no real sense of the 8 hours you slept through. How long a dream feels is IMHO a function of how vivid it was - aka, having more 'data' to integrate into memory.
I am speculating here, but I would imagine that marijuana causes the brain to sort of 'open the flood gates' of sensory input. It also changes how this input is integrated into memory, meaning that one 'unit' of memory data may be integrated into a wider web of synoptic 'memory circuits.' Again, this is all pure speculation, but I would imagine that the increased input, combined with a change in integration, would mean that when stoned, you are integrating more memory than when you are sober, simply because there is more to process, and in a qualitatively different manner.
You know how they say that pot is bad for the memory? I believe it is more that pot causes memory integration to occur differently, such that when sober, retrieving these memory circuits for recollection is impeded because retrieval is somewhat dependent on context. And similarly, pot encourages creativity, as it broadens this process of integration - allowing one to look at something in a much broader, and less routine sense. The context of being sober is different from being stoned. (also why they say that if you study high, take the test high.)
If someone actually knows the answer, I would love to know.