First find time period
[tex]\\ \bull\tt\longrightarrow Frequency=\dfrac{1}{Time\:Period}[/tex]
[tex]\\ \bull\tt\longrightarrow Time\:Period=\dfrac{1}{Frequency}[/tex]
[tex]\\ \bull\tt\longrightarrow Time\:Period=\dfrac{1}{256}=0.003s[/tex]
Now
[tex]\\ \bull\tt\longrightarrow No\:of\:times\:it\:vibrate=5(0.003)=0.015times[/tex]