Actually, it is 100% possible to have a dream that you perceive as being days or weeks long. I had a dream that seemed to last three days and two nights, but I could only remember enough highlights of those days to fit into about an hour of total time. You can really only dream as long as your REM cycle lasts, but you may skip around in time(for example, taking 5 seconds to walk a mile) and feel like you've been dreaming for days. Personally, I haven't been lucid enough grasp time perception while dreaming yet either. It's a bit complicated.