What is the "time constant" of voltage decay in the soma?

[Christopher Anthony Del Negro]

the neuron's voltage will discharge with a time constant tau equal to the
input resistance * whole-cell capacitance. so if you assume to have a
spherical cell (isopotential) the geometry will allow you to compute the
surface area of the cell and it's capacitance will be 1 microfarad/cm^2.
for input resistance, if you are using H&H formalisms for the ionic
currents you can use the reciprocal of the leakage conductance for input
resistance. 

for more on modeling consult Koch and Segev's "method's in neuronal
modeling" MIT press, 1989.