Why does inflationary expansion of the very early universe require time duration? If the grand unified force symmetry breaking separation into the strong force and electroweak force leads to a change in scale of the metric that defines distance within space is the cause of the inflationary (exponential) expansion of the universe, then why does this expansion require a time duration, ~10^-32 seconds? Why would it not be instantaneous?
I am looking for a layperson answer, but I would also appreciate the real answer, even though I probably do not yet have the knowledge to comprehend. The motivation for this question is that since the inflationary expansion is superluminal, what other restriction exists that would prevent this expansion from being instantaneous?
 A: 
If the grand unified force symmetry breaking ... is the cause of the inflationary (exponential) expansion of the universe... Why would it not be instantaneous?

That is a big "If" out there bro/sis.  Nevertheless, let's just suppose "grand unified force symmetry breaking" is "the cause of the inflationary expansion". 
In layman's terms, the explanation is simple: can you boil a kettle of water from the liquid phase to the vapor phase instantaneously? If so, pls let me know pal, 'cause my coffee machine could use an upgrade.
In non-layman terms, we are talking about the transitioning between two phases:


*

*The GUT phase, where the all forces are grand unified  under a single group, being it $SU(5)$, $SO(10)$, or the surely-right GUT theory engineered by my grandma. 

*Symmetry breaking phase, where we have "separation into the strong force and electroweak force".


The key word here is "transitioning": you have to evolve between the above two equilibrium phases. The transitioning is an inequilibrium process, which takes time. 
A: I found this paper on inflation
Inflation models try to correct the original Big Bang model problems:

Despite its successes, the standard big bang (SBB) model had some longstanding shortcomings. One of them is the horizon problem. The CMBR
  received now has been emitted from regions which never communicated before
  sending light to us. The question then arises how come the temperature of
  the black body radiation from these regions is so finely tuned as the results
  of the cosmic background explorer (COBE)  show. Another issue is the
  flatness problem. The present universe appears almost flat. This requires
  that, in its early stages, the universe was flat with a great accuracy. Also,
  combined with GUTs which predict the existence of superheavy monopoles
  , the SBB model leads  to a catastrophe due to the overproduction of
  these monopoles. Finally, the model has no explanation for the small density
  perturbations required for the structure formation in the universe  and the
  generation of the observed  temperature fluctuations in the CMBR.
  Inflation  offers an elegant solution to all these problems of the SBB
  model. 

The paper  is mathematical , I will copy the conclusions:

We summarized the shortcomings of SBB and their resolution by inflation,
  which suggests that the universe underwent a period of exponential expansion. This may have happened during the GUT phase transition at which
  the relevant Higgs field was displaced from the vacuum. This field (inflaton)
  could then, for some time, roll slowly towards the vacuum providing an almost
  constant ‘vacuum’ energy density. Inflation generates the density perturbations needed for the large scale structure of the universe and the temperature
  fluctuations of the CMBR. After the end of inflation, the inflaton performs
  damped oscillations about the vacuum, decays and ‘reheats’ the universe.
The early inflationary models required tiny parameters. This problem
  was solved by hybrid inflation which uses two real scalar fields. One of them
  provides the ‘vacuum’ energy density for inflation while the other one is the
  slowly rolling field. Hybrid inflation arises ‘naturally’ in many SUSY GUTs,
  but leads to a disastrous overproduction of monopoles. We constructed two
  extensions of SUSY hybrid inflation which do not suffer from this problem.

Scanning through the formulae, it is evident that the theory in all its different  forms, depends on time $t_{initial}$ to $t_{final}$ which is entered in the solutions of the differential equations for the models.
You ask:

then why does this expansion require a time duration, ~10^-32 seconds? Why would it not be instantaneous?

There will always be a time interval for the inflation models, by construction. This interval , depending on the form of the theory, depends on the problems of the Big Bang theory the inflation models try to solve, as described in the first quote.  i.e.
1) The horizon problem
2) the flattness problem
3) the monopoles problem
4) the small density pertubation
These four are made consistent with a Big Bang model when inflation is included.
The interval  $t_{initial}$ to $t_{final}$ has to be large enough to solve the above four problems with the original Big Bang model. It cannot be zero, i.e. instantaneous that you ask, because it would not solve the four problems.
Let me give you the example of the small density pertubation problem, the fact that the Cosmic Microwave Background radiation is uniform to the level of $10^{-5}$  but has inhomogeneities below that, which can be identified with the location of clusters of galaxies and galaxies. Without inflation models, it cannot be explained. With inflation models it is the inflaton field that homogenizes everything but because of quantum mechanics there are quantum fluctuations that are the small inhomogeneities observed. These need a time interval to appear, by definitions of the term "fluctuation". (this is modeled with solutions of quantum 
 mechanical differential equations at   $t_{initial}$ to $t_{final}$ )
