The majority of scholars hold the view that the time of Zuhr comes to an end when the shadow of an object is the same as the length of that object. For instance, if you were to plave a stick in the ground. When the shadow of the stick is equal to the stick’s length, the time for Zuhr prayer will be over.
This opinion is followed by the Mâlikî, Shâfi`î, and Hanbalî schools of thought.
This opinion is found among Hanafî scholars as well. It was held by eminent scholars like Muhamamd b. Hasan, Abû Yûsuf, Zafar, and al-Tahâwî.
The second opinion is that the time of Zuhr comes to an end when the shadow of an object reaches twice the length of that object. This opinion is related from Abû Hanîfah and is the official position of the Hanafî school of thought.
As for the time of the `Asr prayer, scholars are agreed that it starts immediately when the time for the Zuhr prayer comes to an end. This point is agreed upon by all.
It should be noted that since the motion of the Sun across the sky is a continuous one, the time when the shadow of an object equals the length of the object – or for that matter when it equals twice the length of the object – is only a fleeting moment. This means that the time of `Asr comes in immediately when the time of Zuhr comes to an end.
This is what is indicated by the hadîth where `Abd Allah b. `Amr relates that Allah’s Messenger (peace be upon him) said: “The time of Zuhr is from when the Sun begins to make its descent and to when the shadow of a man is as his height, as long as the time for `Asr has not yet arrived.” [Sahîh Muslim]
This same hadîth is also clear evidence that the opinion of the majority of scholars regarding the end of the time of Zuhr is the correct opinion.
And Allah knows best.