The problem here is that "rolling in" will change as you grow... Meaning that when I had 100 subs, I would be getting 1 stub a day, and I wanted more. Then I started getting 6 stubs a day, and I was satisfied for a bit... Then again I wanted more. And still to this day, I wouldn't consider my sub's as "rolling in". And I think this will continue for the rest of my YouTube career.
"Never be satisfied. Always strive to improve no matter how good you think you are!"
However if you are just talking about steady consistent growth, I noticed it at about 2k subs. I averaged 10-15 per day.