I think that they give you the length of time that they do due to all the other stuff you are supposed to do. We have 4 hours contact time a week in 3rd year and odd supervision meetings here and there. Our weekly reading for modules is usually several 100 pages fr each. We then have presentations that we have to give for each module, which take a fair amount of time to prep, as we have to run the whole seminar that we present on. A lot of our tutors like us to hand in detailed essay plans for the essayd for that module before we go on break. A lot of us on top of that then have interviews for further uni courses or jobs. Whilst I think a dssertation of 12,000 words (for us) can take a month of two, with everything else that has to be balanced around it your time just gets eaten up (or mine does anyway lol). I am relieved I don't have to hand mine in until March.