When does your temp dip before AF?

Sarah 👼🏻👼🏻🌈👼🏻🌈
BBT n00b here!
I'm 6 DPO and my BBT is steadily rising. I know it's too early to tell anything - I'm just wondering if the dip is usually pretty consistent in terms of days before AF (and if so, how many?).
Thanks!
Sarah