If you leave the ginger in the fridge for too long, it will become hard as a rock. Then, it will be even more difficult to grate. So, 10 to 15 minutes is the perfect time. Put any leftover ginger back in the fridge, otherwise it will become soft again.
See? It’s that simple. From now on, don’t worry about grating ginger. You can get the job done in 10 seconds.