In all honesty, I'm not too sure: they do do a full report, and I expect it depends on things like the time of year and how busy they are on top - I would imagine you'd get it within a month, though I know our students sometimes get them within about ten days. You're best off getting the ball moving as soon as you get there - either speak to your department's disabilities' rep (if there is one) or go straight to the student advice desk and ask what to do next. It's well worth doing, because as well as extra time for exams, there's loads of useful support you can get - good luck!