I don't think there really is an "average". It really varies from brewer to brewer and some people change thicknesses batch to batch. I have heard of people mashing as low at .95 l/kg and as high as 1.89 l/kg, perhaps higher. Some people with thinner mashes have been getting better efficiency (because there is more water to extract the sugars), but some people have good success with thick mashes as well. Your mash time also has an effect on your efficiency and how fermentable your wort is.