Either they ask you to speak informal with them or they are younger than you. Or same age but you are comfortable with each other not like meeting for the first time
#Formal and Informal
brave thorn
lethal notch
Informal: Close Friends, Close co-workers, Close Family (like siblings), Younger than you.
Formal: That one friend you barely speak to but still refer to them as your friend, Co-workers, Family (Parents, grandparents), People older than you, First time meeting someone
wise trench
If the person is older than you then it's until they tell you to speak informally. If they're younger, then you have to address it. If you're the same age then both of you have to come to an agreement. As for how long you have to know them, there's not rule for that. It's all up to the individuals and the situation.
When I was introduced to my cousin's husband for the first time he told me to speak to him informally right away although he is about 10 years older than me. So yeah, it all depends.