I don't think we'll see him use his behelit until the very end of the manga.
It would be very shocking if we see Guts turn into an apostle, mainly because he would have to sacrifice someone close to him. Plus, we don't know if someone who is branded can become an apostle, but either way, Guts can stand up to them on his own.
I honestly think the manga would not be as appealing if turned into an apostle. The fact that he is a human (albeit overly powerful) is one of the greatest appeals. He has no powers. He has a huge sword, awesome armor, and an insane amount of battle experience.
I think when he finally uses it, it will be for the "final showdown" where they take on the God Hand. As for Guts using it for anything but that, I do not see it happening.