You wish to produce an image with a 8 cm FOV and 256 phase encoding gradient steps. The maximum phase encoding gradient you can produce is 1 G/cm. What should the width of the phase encoding gradient be?

Gmax dt = N / (2 g FOV)

where
N = number of phase encoding steps
g = gyromagnetic ratio for H = 42.58 MHz/T
FOV = field of view
Gmax = maximum phase encoding gradient

If we assume a square phase encoding gradient pulse the equation reduces to

t = N / (2 g FOV Gmax)

where t is the duration of the gradient.

t = 4 ms