I'm on sustanon testosterone, this is my 5th day on T, how long does it take for a voice to break (become deeper)

For some FTMs, the change can begin in a matter of weeks. But you don't have to rely solely on hormones to effect the change you want. Through commitment and practice, one can masculinize his voice through training. Choose a masculine model, someone you hear often that has a voice that you would like to emulate. It helps if your model does not have a voice that is too terribly deep. Listen to how they speak. Note where your voice resonates as you learn to speak in a deeper timbre. Most importantly, believe in yourself as you project yourself through your voice. Note that MTFs get no advantage via hormone therapy to feminize their voice. They pretty much HAVE to rely on voice training.