You got that right, 10–15% is a single girl total story. There will be at least 10-15% story progress of each girls between major update. However, the 0.1 release will be an exception. This release will add some missing features /mechanic that will be important to trigger some event later in the game and this release will also move from alpha to beta state. I don't enter into details to explain which features /mechanic will be add, but I will do a devlog post about it before the 0.1 release.