In my experience you need to have a minimum of 100,000 to 150,000 unique monthly visitors to your blog or website in order to start earning a part time income blogging.
Anything under 100,000 unique visitors to your blog you could still earn a little money but not much with Adsense depending on the niche your blog is in.
I started getting $300.00 per month from Adsense through my blog after I reached 150,000 monthly unique visitors and when I got to 300,000 monthly unique visitors I started making $1,000.00 per month.
It took my blog 7 years to get to 300,000 monthly unique visitors and earn $1,000.00 per month.
Blogging is hard work and you have to write on topics that get a lot of searches per month so you can gain a lot of traffic from your blog content.
You can use google ads keyword planner to get an estimate of the amount of searches for a keyword that you plan to write on.
After I researched keywords and started writing on those keywords and topics my search traffic from both Google search, Yahoo Search and Bing really started to increase and with that increased targeted traffic I started getting more clicks on my Adsense ads which earned me money.
I now make money in my sleep from blogging from articles I wrote years ago.
So keep at it and you'll eventually get there if you stay motivated.