Investing in bitcoin is not just about the amount of money you earn, and it doesn't mean you must be rich first; the most important thing is just to have income and understanding.
The understanding one has about bitcoin is what can make investing in bitcoin easy and successful. When there is no understanding, one might wonder when is the right time to invest or what is the right amount of money that needs to be invested, but with understanding, one knows how to go about investing.
I think as long as you have an income in real life and a basic understanding of Bitcoin, then you can start investing, provided that your focus is long-term, don't think about becoming a trader who might just waste your time and money. Then, you just need to maintain your consistency in investing, and continue to learn everything about Bitcoin, because Bitcoin knowledge is very broad, it takes a long time to really understand it, so while learning you can also invest.