Determine when Strassen's algorithm for matrix multiplication is better than regular MM
If you look at the number of operations, it's obvious that you would do a matrix multiplication using the smallest number of operations. For 2x2 the direct method is best. You calculate for 3x3, 4x4, 5x5 etc. how many operations you need using the direct method, and how many you need using the Strassen method, performing (n/2) x (n/2) products using the fastest method. 

If you look at execution time, that is dependent on the computer where you run it, so the point where Strassen is better also depends on the implementation. You would do the same thing as above, but instead of calculating the number of calculations, measure the execution time.