The only agents that specialize in signing up new Medicare beneficiaries 6 months before they enter Medicare are slime balls from Bankers Life, why in the world would a good agent want to make a client pay for a Medicare supplement that far in advance? They don't even have a Medicare card.
It takes literally 48 hours for a new Medicare supplement to be approved for someone in open enrollment, there is no justification for pulling that crap with someone other than an agent getting paid.
Not that I enroll people that early, but I know of at least one carrier who will not draft the initial premium until the effective date. Which means the person wouldn't pay any premium prior to coverage being effective.
With that said, I typically complete applications whenever the client is ready to move forward. If that's 90 days in advance or one day in advance, makes no difference to me. However, when they ask me when we should move forward, I tell them
"Whenever you are ready, but I recommend no later than 2-3 weeks prior to the effective date. Ideally near the beginning of the month prior. That way you will have your I.D. cards (PDP enrollment as well) in place on the day your Medicare begins."
With all of that said, I think the earliest I've ever completed an app on someone is in the 60-90 day range. And that's the exception and not the norm. Most of them are completed in the 21-45 day range. My conversation typically starts much further out though. Just talked to a woman yesterday whose Medicare starts in October. Probably won't complete an application until late August or early September.