Answer
It would take $1.4 \times 10^9$ years to count that many particles.
Work Step by Step
$6.0 \times 10^{23}$ particles $\times \frac{1 second}{2 particles} = 3.0 \times 10^{23} seconds$
We are going to divide that work by 7 billion people ($7.0 \times 10^9$)
$\frac{3.0 \times 10^{23}seconds}{7.0\times10^9} = 4.3 \times 10^{13} seconds.$
$4.3 \times 10^{13} seconds \times \frac{1 hour}{3600 seconds} \times \frac{1 day}{24 hour} \times \frac{1 year}{365 days} = 1.4 \times 10^9$ years.
