How can one calculate how long a sequence will take?

The total time for image collection is TR x N x n, where N is the number of gradient steps in the phase-encoding direction and n is the number of averages. For a sequence with a TR of 2.5 seconds, a matrix size of 128 x 256 (phase x frequency), and 2 averages, the total time for image collection is 640 seconds, or 10 minutes, 40 seconds: 2.5 x 128 x 2 x 640. Changing the TR to 0.5 seconds changes the total time for image collection to 2 minutes, 8 seconds.