Chaga produces annual growth layers. These can be difficult to identify but can be seen in some conks if they are carefully split down the middle. In this particular conk I was able to identify 15 growth layers (marked with black lines in the photo) suggesting this conk was over 15 years old. It weighed about 1kg when it was dried or about 2.3kg when it was fresh. This is consistent with other data I've collected which suggests that chaga grows between 150g to 200g per year (fresh weight)
It is common for conks to grow double this size. In some cases chaga can take over 30 years to reach maturity.