"The Majority", so I guess at least 50% :)

> Currently, new revenue from YouTube Premium membership fees is distributed to video creators based on how much members watch your content. As with our advertising business, the majority of the revenue will go to creators.

From: https://support.google.com/youtube/answer/6306276?hl=en

EDIT: found this: https://www.reddit.com/r/PartneredYoutube/comments/9mj13u/sh...

According to them, a view from a Youtube Premium user nets them 36x more money than a view from an Ad viewing user.