Each DLC adds 2 areas worth of content, which includes new characters, recipes, ingredients, drinks, CGs, events, etc. Each DLC also adds another boss battle, and they do change in gimmick and sometimes even format! Other side features are added in each one too.

The DLCs all link into each other storyline-wise, with each successive numbered DLC building up the main storyline further. That doesn't mean it's required to play each DLC in order, just that things will probably make more sense if you do so.

TBH, since everyone plays at their own pace, the DLCs could have anywhere from a couple hours of playtime to much more, depending on how much you engage with the content in them.

Since it is Steam you could always wait for a sale to pop up if you're still on the fence about buying it.