I doubt there is a standard for such a measurement, it'll vary by car. It will probably vary between drivers too, since steering wheels can often be customised to their own specifications.
I doubt if there will be any definitive answer to this. Whatever feels right I suppose. You can look at photos of real wheels and try and gauge it from there. I know that on TM wheels the paddles are too close and my fingers get stuck. I've just measured some custom wheels I have any they vary from 28mm to 40mm from the back of the wheel grip. I would say somewhere around 30-35mm would be perfect.
I agree with you guys, there are many different options. I've bought the Ascher Paddle Shifter to upgrade my old Fanatec Formula steering wheel and with that there is a much bigger distance to the paddles: (about 4cm to the steeringwheel) It felt unnatural at the beginning because I'm used to the 3cm distance with the Podium paddleshifters (on the down right side). That's why I'm wondering what are the common distances in real race cars (never thought about that before). Looking at the 2019 Porsche GT3, it even looks like 4cm paddle distance is quite possible: