I tried to get in touch with my BGO for several months by leaving him messages/sending him emails. When I finally got in touch with him, it turned out he wasn't my actual BGO-just the BGO area coordinator who gets me in touch with my actual BGO.
So by then it was the beginning of August and I tried getting in touch with my "new" BGO by leaving him messages/sending him emails as I had prior. When he finally answered my emails, which took him quite some time, he told me he was out of the country for 3 more weeks on some type of Navy training.
So what I'm trying to say is..... it depends. If you have a busy BGO, someone like mine who has a very demanding job outside of being a BGO, it may take some time. Or you could have a BGO who isn't as busy and can correspond with you very quickly, in which case you can get your interview done very soon.