I would aim for about 20-25 minutes. That's also a good update size regularly. Of course more would be appreciated.
Anything under 15 minutes should be avoided.
I like this game, but for me it's definitely not long enough, especially considering its version 0.2 not 0.1
Also, I find games that manage to be erotic in the first version, or find a way to include sex, without it feeling forced, are a huge plus.
I'm by no means a big spender on patreon, but the games I do support mostly did everything right on the first release.