As other BGO's have stated there is no exact rule when an interview will be conducted. I like it when candidates reach out to me via e-mail and introduce themselves and have questions. More than happy to answer questions and point candidates in the right directions. As far as the formal interview itself, i'll schedule when candidate has application 60 to 70% complete.
As a candidate keep this thought in mind. We are looking for kids that are self motivated and dig out most of the answers themselves. At most I'll send out a general reminder regrading dates and timelines but I never chase kids or pester them to get parts of the process completed. If the candidate is not motivated enough to do their own follow through then they unsuitable for success at USNA.