Max Discharge Rate of typical Alkaline Button Cell Batteries
Batteries are complicated chemical reactions. The 30mAh will be when drawing some specific current. A larger or smaller draw will change that value. So will temperature.

What's the maximum current you can pull? Well, the battery has an internal resistance which will limit the current. The formula is I=V/R.

How long will it last at that level? No idea, but it's certainly going to give you less than 30mAh.