the short answer is NEVER.
the long answer, is people are so wrapped up in subscribers, and not concerned enough about the quality of the content they're putting out.
Admittedly in the past I've asked "what's a good amount of time to get x amount of subscribers" until I realised how rediculous that question is because we all take differant amounts of times to get anything done.
How long did it take you to learn how to drive a car? How long did it take you to make friends? some things take longer than others.
Focus on the content first, and don't care about the subscriber count. 100 subscribers is a great goal to aim for, but it shouldn't keep you up at night wondering what a good timeframe is. You'll get there when you get there