For my first side project which has resulted in paying customers the whole process took quite a while. From the idea to first sale the whole process was roughly 30 months (May 2014 -> Nov 2016). There were a few false starts and most of the work was done over the last few months, but I'm glad it's made it to a real release.

What is it? It's a crowd funded user interface rewrite for an open source musical synthesizer: http://zynaddsubfx.sourceforge.net/zyn-fusion.html

Challenges:

- Bug testing and regression testing a large/complex user interface was difficult as it tends to be a largely manual process

- Maintaining a multi platform codebase added a lot of extra complexity and it certainly made distribution more difficult

- Identifying what the users precisely needed was difficult as time could be spent on a number of different facets of the project

- Polishing up pieces of the application after it was functionally complete took much much longer than originally predicted

- Marketing/Estimating-the-market: Initial surveys indicated a large number of users would be willing to pay to support a replacement UI and a few hundred were on a dedicated mailing list for it. Conversion rates were much lower than expected with both the general audience and those subscribed to the sub-project specific mailing list.