Been thinking a lot the past couple of weeks (guess the new year has this effect on me) and I think $5000/month is the number.
Of course it also depends on a few other factors like the stability of the recuring revenue (not just a one time sales spike) and if it's something I can see myself working on it longterm - but simply put - $5000/month is my target.

It really depends on the situation, but if I stay in Hungary (low expense home country) then if I can make $4k/ month constantly I would never consider going back to work. After taxes it's about 3k which is the maximum you can earn here as a developer after taxes so it means having a really nice income compared to the expenses. If I move somewhere else, this amount will change. But I left my job happily when I reached the point to cover all my expenses which was back then 1k doing a little bit of freelancing. Now I have much more time working on reaching this goal, which is invaluable cause all of us have 24 hours a day, slicing of 8-9 makes shipping much harder.