Depends on how dark you take the roast, I usually lose about 16%. This on Google Roasting - The Coffee Wiki
I do 1 kg lots in my Garanti and I lose 10 to 15 % roughly. I used to record the weight loss but I don't bother anymore.
15%-18% depending on the roast depth etc
Over the last dozen or so roasts I've done on a behmor, I've had between a 15 and 18 percent weight loss, with an average of 16.2% (These are on roasts that are usually around 300g of greens).
I typically roast 300g batches and they're usually ~250g when they come out, so approximately 16%.
Exact figures here:
Ethiopian Limu 1.0 kg to 0.808 kg = 20% loss
PNG Wahgi 1.065 to 0.885 = 15% loss
Cuban Altura 1.0 to 0.8 kg = 20% loss
Indian Monsoon Malabar 1.0 to 0.814 = 19% loss
So yes Vinitasse is correct, sorry about my 10%, it's too low. I looked back over my notes so it looks like 15% to 20% is normal. I take my beans to just the start of SC.