How long do I need to feel like a boy before I can be sure enough I won't change my mind (especially since I'm the type of person who does that a lot) that I can ask my college friends , who I'm sure would go with it to call me "he" and my "boy name" . I'm on they/them pronouns now because being called "she" bothers me but I'd sort of like to be called "he" . My birth name doesn't bother me right now but I'm pretty sure I can't expect people to think of me as a man if I have a girl's name.

Hun, I know this is going to sound like a total cop-out, but the answer to your question really is: "when you feel you are ready".

Some people feel ready the very instant they realise they're trans.
Others take weeks, months, years or even decades.

Maybe explain to them that you're not completely sure about it but want to try it out.  If you try it with a small group to start with, you can always say later on "actually you know what, I don't feel like this is really me" if you change your mind.

That would be my advice to you hun.  Hope I've helped <333