I co-founded a company developing QA automation software in 2012. Worked on it full-time until 2014. Had an EU grant covering our costs during that period. In 2015, I invested about 200h into it (mostly answering support emails and dealing with taxes) and made about 20k€ this year.
I've been working on an Appointment Reminder clone for a year now in my home country Austria. I will about break even in 2015, but will have an MRR of ~1500€ in 2016, with low ongoing costs.
So I reckon that in 2016 I can have a pretty passive income (working 5-10hrs/week) of 2000+€ per month. Not that I will work that little because I obviously still want to grow my income. And I also need to add that I've been earning considerably less in the past 3.5 years than I would have if I had stayed employed as a software engineer.
Oh, and then there's the Android app that makes about 15€ per month ^^