Don't be daunted by this word - not that much is expected of you. Your initial audition will probably last ten to fifteen minutes but most directors hold recall auditions and you should ask the director when and where the list of people selected for these will be put up - they are generally put up on the noticeboard outside the ADC Clubroom, which is where the cast lists usually go up as well. If you're not recalled or cast in anything, don't take it personally and keep on trying. Auditions are very hit and miss and some of the most successful actors in Cambridge started off by being turned down by every director they auditioned for. Recalls often involve groups of people and last between 30 mins and an hour and a half.
Different directors do different things in recalls.
Just go along with whatever the director wants and remember you're there to be seen so don't err on the side of caution. The intensity of your rehearsal schedule depends on how big your part in the play is and the amount of time left before the opening night. Rehearsals do not usually interfere with lectures or supervisions. The week or fortnight of performances is the pay-off for weeks of hard work and stress and should be fun. You may be required to turn up an hour or two before each performance to sort out costume and make-up and to warm up.