Major patches (ie. 2.X) are every 3 months. They usually toss in some things that have either been requested or they finished and are able to implement early in minor patches (ie. 2.2X) that often appear midway between major patches (~6-7 weeks after a major patch). With 2.25, while they haven't given a specific time frame that I'm aware of, it's generally assumed to be the beginning to middle of May. From what I recall the only hard information they've given us about that patch is that it will extend the zodiac weapon quest that was released in 2.2.

SE has said that they will begin talking about the first expansion in more detail after the game launches in China (this summer). I'm not sure where they said every 2 years, but I've seen people mention that time frame and it seems fairly reasonable. Might be a bit sooner or later. They've stated that they're working on it already in any case and that they're aiming for it to include as much content as 2.0 did upon release.