Personally I hope its as soon as possible, otherwise, the hype will die down, if we are waiting for a year just to play legacy servers the same time as the new expansion comes out it wont be as hyped as it is right now.
Literally, every single popular streamer has said they are going to stream Vanilla wow, including Summit1g and shroud, the sooner these servers are up and running the better imo.
This will give people time to level up and the economy's to build up on servers and server comminitys and guilds to form. so that when the expansion comes out it will still be new and fresh and alot of people will play the new content instead of just rolling toons on classic servers.
I dont really think its that much work for blizzard to get them up and running and they are trying to launch classic alongside the new expansion.