IMO Ramon Allones & La Gloria are the longest maturing Havanas, as a whole. I would throw punch in there as well, but I haven't quite experienced a "mature" level that they hit at a certtain age. I think there's not a wall like with these two brands that just screams, ready.
That being said, I don't think there's a magical time at which they become "smokeable". It varies for each vitola & how it was packaged.
Before RASS came in cabs, I would buy DB's & put them in CORO cabs so they'd age in a varnished SLB instead of just a normal box. Have yet to crack any boxes I have that way, but it's been years since the first & I have no intention of testing my theory yet. I'm thinking 10 years will be a good time. They should still be tannic but very refined. I had 98's that were just in a box & as of the begininning of this year when I finished them, they were still definately strong, w/ that tingling sensation in your nostrils after you exhale. Yum...