There is currently no way to reliably predict when an earthquake will happen, its strength or length. Earthquakes can vary in their magnitude, the size of the earthquake at its source, and length, lasting from seconds to minutes.
Research has shown, that shaking of an earthquake displays a characteristic pattern. After the first tremors start, building up in intensity, a peak is reached, followed by a fading shaking. Large and small earthquakes start the same way, but there is no way to say when the peak, the maximal magnitude of the quake, is reached.