How about never? Please explain me, why should something that thrives under centrality accept something as Bitcoin that goes against all their values? It's not only central banks, also regular banks and the government itself. Doesn't make any sense.
Basically that. I don't even know why people are still thinking and hoping for banks to adopt Bitcoin. It's like fire and water. There is no way to combine these things. The only thing banks may be interested in is their own version of the blockchain for inside usage, that's all.