If you go by history, excluding the usual final tier, then pretty much all raid tiers lasted 4.5 to 5.5 months since well, Mists of Pandaria, the only exclusion ever was Eternal Place.
Terrace/Moguvault lasted 160 days (5.5 months)
Blackrock Foundry lasted 141 days (4.5 months)
Emerald Nightmare lasted 119 days (4 months)
Nighthold lasted 154 days (5 months)
Tomb of Sargeras lasted 162 days ( 5.5 months)
Uldir lasted 139 days (4.5 months)
BoD lasted 168 days (5.5 months)
Palace lasted 197 days (6.5 months)
You can also look at normal/heroic clear rates on warcraftlogs. About 90% of all logged kills on heroic happen within the first four months. That holds true for Nathria, Sanctum, Eternal Palace, and likely any other ride you will check. Why? Because people usually are decked out in gear somewhere between month 3-4 (esp. with m+). Then you farm the final boss for a few instances, and you're done.
If you now have a blizzard dev stating that "6-8 months is the ideal duration", then that's a pretty bad and sad statement - it's practically not supported by any data, wether it's past raid duration or actual clear rates.