Is there a link between 'dream time' and sleep length?
There is no clear relationship between length of sleep per se and 'dream-time'; it would depend on a number of factors, including (and especially) how long you would spend in a particular stage of sleep (REM sleep, for example, where we most often experience dream content).
However, even spending longer amounts of time in REM sleep would not necessarily mean that your dream would seem 'longer'. Because of the bizarreness of our dream content, and from what we know of brain activity during REM sleep, the processes that keep track of time and the sequential ordering of events that take place during waking experience are unlikely to be similar.
One exception to this however, might be lucid dreaming – a rare ability where the dreamer can control dreamed actions at will.