If you've ever played Spankquest or Diary of a Slave, they did really well on the length for spanking scenes. About 5-10 slaps, then some descriptive text or dialogue, repeated 4-5 times. If you're making a spanking focused game, do not worry about being repetitive. Trust me, the more the better lol. From what I noticed in other games, and peoples feedback/opinions on them, people REALLY enjoy having both the option to do the spanking, or receive the spanking. While one sided spanking games are fun, you'll attract more people if you build a game that allows you to do both. Either it be a setting you choose at the start of the game, options you select during dialogue, or just stat based/random events that can trigger.
Regardless, I enjoyed the demo you made, and I'm excited to see what comes next :D