Whatever you want to write
and fits the gameplay, story or slideshow. If the MC is single, there are no one night stands and dating is a core element, it makes sense to wait. If the MC has an LI, not having sex for months without an explanation is gonna be found a bit weird.
Oh, and some people don't like sex with an LI in the first 10 minutes or so. They want to get to know a character a little first. That's a personal preference though, you can easily ignore it if you want.
if you want to tell a story, sex must be late. when you give it up you've reached a peak of your story. don't waste your most important dramatic asset in the beginning.
if you don't care about stories you can start spamming sex as early as you like. "Hey, lady, did you order a pizza?" bow-chica-wow-wow.
both have their fans.
That makes sense for a story where sex is core of the plot and vanilla sex is the peak. But it's not hard to imagine something story-based where sex comes early. It just needs a plot about something other than sex or something more than vanilla.
There will be people complaining about dream scenes.
I think it's good to understand
why people complain about dream scenes. Nothing that happens in dreams normally has any consequence. If a dev uses dreams to show stuff without consequences, like to spam early meaningless dream sex scenes, that's pretty boring to read. It's often a good view into the fetishes the devs likes, though.
But if it has some relevance for the story, like a dream world, prophetic dreams or insight in the MC's mind for other things than "the MC is a horny fucker", that's more accepted and doesn't get as many complaints.