I'm not looking to share a ton of information about the first update right now, however...How many new days will be in the update? 50? (JK)
What's probably most interesting to you is playtime length, right? The way I do day count pertains to the flow of the story and fitting things in a more "plot progression" way. In other words, the reasons why I would do ten days versus five days is for story purposes, rather than it being about content length, and I'm guessing content length is what's most important to the majority of people.
So to answer, the day count of the first update isn't locked in yet. Likely 4 or 5 days, but the playtime length will still be substantial... likely around 65-75% of the length of this first release.
With that said, there's about double the sexual content, and a very cool feature planned that I think most players will love.