The Chicago Sun-Times has an article on the composer, which mentions Harry Potter and the Prisoner of Azkaban:
Williams then starts composing, working on a schedule he describes as "always too little time." Before arriving in Chicago, Williams was in London catching an early glimpse of the third Harry Potter film, which he is scoring for scheduled release next summer.
"The principal photography is ending about now," he said "I'll record the music in March, so that leaves about two-and-one-half months to write almost a two-hour score. It's colossally tight. And after mid-March, there's another month or six week for mixing and dubbing. So I would say for a picture like that -- fairly long, with special effects -- we're talking about four or five months.''
Thanks to Sue for the link!