There has been no generally accepted standard of the minimum level of a R2 before it can be considered as large. Nevertheless, "large" is a relative term. That is, we need to have a counterpart to compare before we can say it is larger or not. A R2 of 0.1% can be large if the best current R2 achieved in other studies is only 0.01%. Likewise, a R2 of 90% can be small if the best current R2 achieved is already 99%. So, one needs to look up what others have achieved before we can tell if we are doing better or not.