I don't know how often Theology interviews, but most PhD programmes do. There's no set length of time for the decision process. Applications that are very clearly not up to the standard, or are very clearly never going to be beaten in that year's application pool will get quick decisions. However, most applications are 'there or there abouts' and so the admissions wait until they have a clear view of the quality of the applicant pool. It's also something of a matter of detailed topic, you don't want to offer an early place to an OK applicant who wants to work on St Cuthbert, because you might get a superb person later in the application cycle, and you can't have lots of people studying the exact same thing (lack of Supervisors, books etc). So if you are 'there or there abouts' then three months is about the average wait, most decisions are made by April.