Warlords of Draenor classic is going to last 5 months.

At this rate, Classic might catch up with retail by 14.0 ...then they can start the cycle over again.
Edit: Let's theorycraft this. If length of classic expansions correspond to their original popularity:
TBC Classic - (June 2021) 15 months
Wrath Classic - (September 2022) 20 months
Cata Classic - (May 2024) 11 months
MoP Classic - (April 2025) 10 months
WoD Classic - (Feb 2026) 6 months
Legion Classic - (August 2026) 10 months
BFA Classic - (June 2027) 5 months
Shadowlands - (January 2028) Classic - 6 months
Dragonflight Classic - (September 2028) 8 months
TWW Classic - (May 2029)