Answer
$t=0.75~s$
It represents the time it takes for the pressure to complete an entire cycle.
Work Step by Step
The period of $cos~x$ is $2\pi$
If $x=\frac{8\pi t}{3}$:
$\frac{8\pi t}{3}=2\pi$
$t=\frac{3(2\pi)}{8\pi}=\frac{3}{4}$
$t=0.75$
