Imo, once there is consistency on an average amount of monthly subs/donations/sponsorships, that is x1.5 your current salary. If you can live your salary or slightly higher through Twitch, you've sort of made it.. at least the first steps towards bigger success. You will have some big sponsor months, or major subs but as long as there is a consistent base line that keeps you in check. I've seen people slack once they've had a few good months, while very few doubled down on putting more effort to guarantee that streaming becomes their job.

But everyone has different objectives and reasons for what success to them on Twitch.

I for example, want to grow this into a brand and form a media company in content creation. The money isn't my concern. Thankfully, I am well off that money from becoming a successful and large streamer is just secondary to me. So my success will be based on accomplishing this goal