I don’t know of an official date. There is an announcement for a new client software that supports end-to-end encryption on Nextcloud. For this feature, you will probably need the new client:
It is announced for NC 13 which is end of the year. The ownCloud client probably stays compatible, it’s hard to say when they introduce something that breaks the compatibility completely. I would switch next year when the new NC-client is working without major hiccups.