Too many factors! Depends on how many domains you sell and what your profit margin is, vs the sites’ earnings.
If you get a site to the point where it earns reliable income, it’s a gift that keeps on giving. Don’t think of it as “developing a domain” but as “building a business.”
But you need to put in some effort to make it work, and some people don’t want to be bothered.
Your quality feedback is nice, thanks. ..some people don’t want to be bothered...
I (with no regrets) could have built a full-time income from simply selling domains at retail outlets, but the "don’t want to be bothered" details are not something I would want to do all the time.
The Big Dream = When your source of income matches something you Love to do, then life is great.