etiquette - At what point can I call my professor by their first names as an undergraduate research student?

I'm currently working closely with a professor on a research project as an undergrad student. I always address her by her last name: Dr.xx. However, this makes me feel distant from her.. I am wondering by which point I could call her by first name?

Here is some background info: I got to know her in person by taking a class taught by her and then started working together on this project at the end of last term. We had meetings together for a few times and exchanged a few emails back and forth. She always addressed herself by her first name in the email but I insisted called her Dr.xx to show my respect. However, some of her grad students in the lab are calling her by first name...

Thanks!