To find the model that represents 23% of Salina's time spent on math homework, we need to calculate 23% of 100 squares.
23% of 100 is: \[ \frac{23}{100} \times 100 = 23 \]
This means that 23 squares should be shaded to represent 23% of the total time.
Therefore, the correct model is: A grid model with 100 squares. 23 squares are shaded.