I'd imagine it is different for authors in different phases of the business, starting at very substantial while one does not have a lot of a personal platform and decreasing markedly when one does.

Someone, and I'm forgetting who it was, suggested that paid subscriptions is the natural monetization strategy for people who are a) public intellectuals and b) do a lot of their work on Twitter, and in that sense you're doing marketing with a sort of cycles which might not read as "intentionally doing marketing work." (I think as the community distributes the technology of running these businesses, like the technology of running e-book businesses has been distributed, this will get easier, too. One thing all newsletter authors should be doing ~constantly is hitting up podcasts in adjacent spaces, because it gets juices flowing for writing and because every time you talk to 10,000 people you'll sell N new subscriptions.)