What causes the shielding effect to remain constant across a period?

The small molecule cyclic amp (camp) takes about 0.2 second to diffuse 10 μm, on average, in a cell. suppose that camp is produced near the plasma membrane on one end of the cell; how long will it take for this camp to diffuse through the cytosol and reach the opposite end of a very large cell, on average? assume that the cell is 200 μm in diameter.