There is no one clear answer for this question. But usually it goes like this when a YouTuber starts:
1. They got great appeal and their videos just view right -> YouTuber blows up in the first half year.
2. They got some appeal and their videos are pretty entertaining at times -> YouTuber gains subscribers slowly, and over time ends up in the 1000s (like me myself and I) or even has a chance to hit big numbers given a great enough time span and a shout out here and there.
3. They got no appeal and their videos just don't invite the viewer to finish watching them -> YouTuber generally won't break 100 subscribers, and that seems to be the vast majority. These YouTubers either quit outright, realizing it is not for them or start moaning on forums that they don't grow. A few are like me, not a care in the world, just continue uploading because it is a great passtime. And even at sub 100 subscribers I got a small dedicated following that left comments.
Moral: Don't start YouTube for the numbers.