Oh fun, a math problem.
Let’s say you’re making $1 CPM which is reasonable if you’re focused on having users that come back regularly but maybe low if you’re looking to trick people into visiting (at which point the ads become a convenient exit).
$1 CPM means that for every 1000 page views you make $1.
I think most sites average 5 or so page views per visitor per month. That seems low because it’s coming from Google analytics, which doesn’t differentiate between random visitors and visitors who have accounts. So let’s say it’s 50/50.
You want to make $4k/month because you could live off that (although most people who are capable of making a site that makes that much money are also capable of getting real salaries so maybe the number should be $10k).
Working backwards, $4k would require 4,000,000 page views per month ($4000 x $1CPM).
At 5/views/visitor/month you’d need 800,000 visitors.
Guessing that half of those have accounts, you’d need 400,000 users. Now that I’ve gotten this far, I think those have to be active users. Probably framing this in terms of users is just too hard to answer.
I did work for a site that had 200k users and tried adsense and these numbers were in the ball park.
Some people do much better and my take away is that you can’t just create any site and expect to get make money.