I earn about $2000 a month from ads on the landing page (organic SEO), but very soon plan to add a subscription for pro features (while people could still compile it from source and get the full experience without paying, if they wish).
I started the project 8 years ago to create a slick looking note-taking app for myself on Linux. Then I open-sourced and published it, and it just took off and got popular (more than 1.2 Million downloads).
Took around 2 years to get a high rank on Google. Then it was just a matter of putting ads (which I don't like but my income relies on) and ever since it's been quite stable.