Earning from Adsense depends on number of factors including the Cost-per-Click (CPC) and Click-through-Rate (CTR). If you have content based on high CPC keywords and get traffic from western countries, you can make a decent earning.

For example, if you serve 10,000 page impressions per month with average CPC of $0.50 and a CTR of 3%, your monthly earning could be $150.

In a lot of countries (ex. Belgium, where I'm from) that is not a decent income. Not even close. So you'd have to have a lot of those niche sites to earn a (self-employed) living.

I run two blogs that i kept updating over the years. I don't post frequently, maybe once a month. I earn around 100-150 euro a month from them. It's not much but think of it as pocket money - its a zero effort income - especially if the topics you write about genuinely interest you.

It is supposed to be passive income, so yeah, you have to build a lot of such sites (or a few with high payouts) to replace your regular /salary earnings.

Yeah, that falls into the "just work a few extra hours in the day job and save yourself the hassle" category... Good to know that's how the numbers stack up.