Never, because everyone and their brother owns the same bike, and 50 years from now it'll still be in the same condition.
The Harley's that are worth big money today were 60-70+ years ago treated like todays Metrics - ridden hard and put away wet, never planned to collect it or to even keep it around. They weren't even taken particularly good care of. They were utilitarian. The ones that survived were more because of luck than any intentional effort to ensure such.
IMHO there's a far better chance of the less common metrics from the last 10-30 years being worth more than today's HD's simply because there won't be a lot of the metrics around, while in 50 years...everyones still gonna have their 80's, 90's, 2000++ HD's in their garage - they'll be like old Corvette's - only the actual rare ones will be worth anything, all the rest are gonna be cheap and plentiful.
Things like the Honda Rune, the CBX1000, old BSA's and BMW's and the more obscure but not intentionally collectible metrics from all the other manufacturers will be the ones to watch IMHO.