At the middle end you can pretty much estimate consistent viewers = subs, usually around the 500 to like 3-4k viewers mark. So between 1000 and 10,000 a month which means it can be a little bare bones on the bottom end, but much more comfortable once you hit the 1000 sub mark at which point you're basically at a 30k a year job which a lot of people already live off of. Throw in donations which can be almost no money to much more than subs and you're good to go. Forget advertising unless they do bounties and sponsored streams, because unless you're in a better negotiated deal with twitch and have 10,000+ viewers per ad you're not seeing much at all from them.