I've had a few (rare) dreams where they seemed to last several full days when in fact I'd only been asleep for a couple of hours. As far as lucid dreams go, most the time they're somewhat short for me. This seems to be due more to losing lucidity than waking up (except when I was new to lucid dreaming and would wake myself up from getting too excited). However, I have had a few pretty long lucids. (A couple examples from my dream journal; Wednesday Night, 9-19-07 and Sunday Night 9-23-07 )