That's basically what I did. I don't remember how long it took me, but surely 2-3 years (with breaks involved).
For an open source project with modules/plugins it is very hard to build a good theme. There is not only design, but also a lot of technical aspects (speed, seo and so on). A lot of merchants expect a theme to be 100-200$ and then they are fine for 10 years. Imo that's no more realistic these days. I see a lot of SaaS, that have a higher pricing per month...