1952 ... first microseconds
... there is some confusion about the terminology. One millionth of a second is a 'Micro-second'. ( I think milliseconds [one thousandth sec]sounds cooler and gets used more often. These images are produced by a "Rapatronic Camera" developed by EG&G.
"... At this point in the explosion, a true hydrodynamic shock front has just formed. Prior to this moment the growth of the fireball was due to radiative transport, i.e. thermal x-rays outran the expanding bomb debris. Now however the fireball expansion is caused by the shock front driven by hydrodynamic pressure (as in a conventional explosion, only far more intense). The glowing surface of the fireball is due to shock compression heating of the air. This means that the fireball is now growing far more slowly than before. The bomb (and shot cab) vapors were initially accelerated to very high velocities (several tens of kilometers/sec) and clumps of this material are now splashing against the back of the shock front in an irregular pattern (due to initial variations in mass distribution around the bomb core), creating the curious mottled appearance." via
... in microsecond time- I second would last 11 days. 13 hrs!