For known developers with History of good projects short 20-30min demos can do.
For new developers its important to hook people right from the start so the more you can squeeze and polish in the first release the better, people will unlikely come back if the first release is poorly translated with filler renders and no story beats to hold them interest no matter how much they intend to go back and polish it.
In my opinion if it's a visual novel or something like that at least a couple hours, you need to see if the story and characters are interesting enough, but if the game has actual gameplay I'd say 30 min to 1 hour showing enough mechanics and some scenes it sould be enough for the first release.
I'm ready to play as long as I'm interested, and it doesn't matter how long the game lasts, even if it's 100 hours. The first release should just have an interesting premise, the length isn't that important.
As long as it takes to show the premise and basic gameplay, thats it. If you can do that in 5 mins, then 5 mins. If it takes an hour, then an hour. People should finish the demo and know what to expect for the rest of the game
Title is a little misleading "Game Length" then ask about first release.
I normally wait 3-5 updates after release to check out games/vn's to see if I like it then will not play again until its finish.
To many games/vn's get abandoned.
like other said here already the game should be long enough to leave you satisfied with what you have played but also making you want the next release as soon as possible