Well you don’t HAVE to start using “du”. There are people who have been friends for years who still use “Sie”. It’s a mutual thing, when you feel comfortable with the other person you can tell them it’s okay for them to call you “du” and usually they’ll reciprocate (if they don’t, they’ll tell you and you go back to “Sie”). Or you can take a bit of a risk and just start using “du”, without asking and the other person will follow (unless you gravely misjudged).

No I can’t tell you how to know when it’s the right time for that, don’t ask me about how to behave like a human, I have social anxiety, hah. But normal people can intuitively tell when that moment has come.

Also there are constellations where you use “du” from the beginning and where it would be weird to use “Sie”. Everyone on the internet is “du” unless you know you’d call them “Sie” in real life. Children are “du” and it’s up to the adult to decide whether this 16 year old deserves a respectful “Sie”. Young people tend to default to “du” among each other because “Sie” is stuffy and square.

Mind you, having grown up with this hasn’t helped with learning Japanese because while there are certain similarities (desu/masu corresponds VERY vaguely with “Sie”), there are too many differences for my experience with my native language to be of much help. But if you’d drop the suffix from their name in Japanese, it’s very safe to say you’d be on “du” in German.