My apologies Lytel, I've been taking a short break and haven't been as active lately.
I'd put the length anywhere from 12-20 hours depending on if you're a completionist or not. The highest time I've heard so far is 24 hours from a person who 100% finished the game, but I could see someone rushing through it in 8-10 easily if they wanted to.
On the battle system, Celestial Hearts uses a side-view ATB system (somewhat similar to the older Final Fantasy games, where each character takes their turn immediately after their ATB gauge fills up). It's definitely more engaging than Knight Bewitched' battle system was, but it is still an RPG Maker battle system.
I went ahead and uploaded a demo - you can try the game for free and purchase it later if you like it!