My sons BGO wants to see that at least 50% of his application is submitted before scheduling his interview.
That's a pretty good rule of thumb. The reason is usually twofold. First, the BGO wants to know that the candidate has a reasonable intention of completing his/her application. Lots of folks never get beyond sending their SATs. Second, the BGO wants to know the candidate has put as much effort into the process as the BGO will. It's important to understand that interviews take a lot of time for the BGO -- usually at least an hour for the meeting, sometimes travel to and/from, and then probably another hour to write up the interview. Before the BGO spends those 2+ hours, most want to see that the candidate has spent some of his/her time on completing the package.
The above said, if is perfectly fine to reach out to your BGO at ANY TIME in the process with questions. It's one of the reasons we're here. And some BGOs will do interviews with little to none of the packet complete -- often b/c they want to get them done early. However, as a general rule, I wouldn't make contacting your BGO to request an interview the first item on your application package list of things to do.