A bit over an hour I guess.
But, the meditation high lasts for a long time afterwards. Days even. So there is the obvious option to build the high via multiple sessions.
I get the biggest results by meditating multiple times a day. Like 5 or 6 sessions of "concentration meditation". And also fasting, doing only chill activities, avoiding drama, solitude...
A few days of that delivers huge results.
That said, I only meditate once a day these days. And I don't do concentration, just shikantaza.
"Medication?"
Dang ole autocorrect
Many years ago, as I was just getting started with meditation, I spent a lot of time on it. Occasionally I'd do a session that went several hours (probably 4h max). Usually the sessions would be shorter (up to about 45min) but I frequently did several per day.
I can't really be very specific (it's a long time ago) but the effect was quite noticeable. Like I was grounded in a different reality, much less attached to things in every day life. It felt good.
Nowadays I don't really meditate any more. I hope I can get back into it though...