A physicist is experimenting on an elastic thread to make some observations. He fixes one end of the thread to the ceiling of his lab and attaches a block of 3.25 kg to the other end. This extends the length of the thread by 0.125 m when the block comes to a rest at the equilibrium. He then pushes the block vertically down from the equilibrium position, extending the thread further by 0.250 m. If he releases the block then, after what time does the block reach back up to the equilibrium position?