I think 1.5 years minimum for each expac. SoM felt too rushed with 1 year. 1.5 to 2 years for Vanilla, then same for TBC and then WotLK.
As I saw someone else say in a different post I too think having 1 pvp and 1 pve realm that starts up and cycles through the phases and then into TBC and WotLK would be awesome. They should also keep 1 pvp and 1 pve TBC era server and the same for WotLK. When vanilla is ready to roll off to TBC you should have the option of “stepping” through the dark portal by staying on the server or taking a free xfer to the vanilla era cluster. Similar when TBC is ready to roll into WotLK you should have the option to “hop” onto the boat to Northrend or taking a free xfer to TBC era. When WotLK gets to the end your characters will automatically be ported to WotLK era servers before the servers reset and vanilla launches anew.
This would not really take anything away from existing era enjoyers, as their era servers would still be there. It would give those in the community that yearn for forever TBC and WotLK what they wish for. It would also give those that long for FRESH!!! what they want.
If you missed out in 2019, well… then you get another chance at it. You played in 2019, but just would love to go through the phases again? Go at it!!!
I played in 2019, and when TBC rolled out I made 1 clone of my main a holy paladin that I kept on era and took the rest through the dark portal. I missed out on most of p1 of TBC due to some irl health issues, so I would really enjoy getting to do TBC again. I also really love the vanilla world, I had a blast playing it in 2019 and I still enjoy dabbling on era. But playing on era at the final phase isn’t quite the same as playing through the phases. Like yeah, if you’re on the major clusters there’s still people leveling, but it’s a whole different feeling when the entire server is at the same stage. So please 
Give us rotating fresh starts!!! 
