Is antimatter positron-electron pair creation and annihilation happens instantaneously or not? This kind of transition from matter to energy(gamma) takes time (continuous in microseconds like quantum jump) or it's "instant"?
 A: The most precise answer would be to say we don't surely know, however terms annihilation and creation are referred to those processes which are simultaneous
One way to get your head around it is to think matter no different from energy. This way your question will reduce to "does one form of energy (electron and positron) transforms the other form (photon) simultaneously?". You shouldn't think of a classical example though, otherwise this picture won't give you a good intuition.
In case if you're wondering the lifetime of an electron-positron state

The mass of positronium is 1.022 MeV, which is twice the electron mass minus the binding energy of a few eV. The lowest energy orbital state of positronium is 1S, and like with hydrogen, it has a hyperfine structure arising from the relative orientations of the spins of the electron and the positron.
The singlet state, $^1S_0$, with antiparallel spins (S = 0, M_s = 0) is known as para-positronium (p-Ps). It has a mean lifetime of 0.12 ns and decays preferentially into two gamma rays with energy of 511 keV each (in the center-of-mass frame).

A: If the transition were not instantaneous there would some intermediate state between the photons emerging and electron and positron disappearing. There is no such intermediate. state, so the transition occurs instantaneously. When (and if) the transition occurs depends on the photon and electron quantum fields describing the situation.
