The only information I could find claims that about 22% of reddit is NSFW. They calculated the percentage of NSFW subreddits with >100k subscribers. [1]
That's far from a perfect way to measure this, but it's around the statistic that 30% of the internet is porn. My intuition tells me Reddit probably reflects the internet as a whole pretty well.
[1] https://www.reddit.com/r/dataisbeautiful/comments/f94j0y/oc_...