My total from http://www.watermark-image.com last year has been around 35.000€ and it takes 5-10 minutes of support a day and 30-60 minutes once or twice a month doing bug features or new features.
My new project http://timeblock.com that are funded by that income and that I do not write any code for has generated app. 2000€ in recurring revenue last year although it is not passive yet it is MRR :)