MY answer has to be …. “it depends”.
:shhh:
It’s certainly a different environment now from when i started – lots more traffic, but lots more competition too.
New sites seem to take time to build up decent results from SEO, and obtaining FREE quality links seems to be hard to achieve as well.
So you either throw money into a new site, buying paid links, buying content (etc) … or you pour your time and effort into a site.
If you want to speed up the process then you do both.
So therefore the answer is not as simple as x months effort = y monthly revenue.
Sorry.
If you qualify your question with specific parameters then you MAY get better answers.