You don't have to complete the entire chapter to save the game.

I'm playing the Switch version so I don't know if this is exactly the same on Steam (in theory it should be) but the game auto saves your progress everything the "Now Saving/Kunio" icon is in the upper right corner. Generally it's every screen or I think when you buy items.

Just to be safe, I generally will go into Options and go "Back to Title" to make sure the Switch Cloud save is up to date. I know Steam has one too so I would imagine that would be similar.

Each chapter seems to vary in length but if you're able to muscle through the story, it seems to be probably 30 minutes - an hour. I've been doing a lot of side tracking since there's so many side quests and other things to do. Longest chapter could be 3 with the amount of required help you're suppose to give to the townspeople.