[guld]

That depends on various factors, such as how many users one can attract with their small/medium websites, what kind of content is provided by your website, and so on. If for example the content on your website is compatible with an ad-network such as Google Adsense or Amazon Partnernet and you have like 500 users a day, you may earn enough money to pay the server/domain cost of a simple project.

At around 3-5k users a day things start to get interesting.

[gardenhedge]

What happens at 3-5k users?

At what point is it worth creating a self-serve platform for advertisers? Or is that not useful at all?

[guld]

Depending on your content you gain enough traction to expect a steady income with your website (50k-100k yearly or more). That can be enough to focus full-time on the project if you have enough ideas and potential to grow the project, and enjoy working on it.

[iceburgcrm]

When you can't manually handle the updates.

Put a link to advertise here and manually manage it until you have too many ads to manage yourself.