Ciri is in the middle of training while Geralt repairs his armor. She struggles with her technique and becomes frustrated that she isn’t executing as she expects. Geralt calls it for the day and notes that any more training will just suffer from diminishing returns. The implication is that she can keep training, but that each additional amount of time allocated toward training would yield smaller gains to her ability. Ciri would be better off focusing on rest instead.
Thanks to @EconWoodrow for the find!