The time required to make a theme has been quickly escalating. I think the two weeks development time you read on the old forum was from years ago.
Even with a team it may take at up to 2 months to make a theme that is acceptable on this marketplace. The reason for this is due to the sharp increase in quality standards and the rise of the mega themes.
If you make a simple niche theme, it will need to either be picture perfect or have some really compelling features. Which I turn increases development time.