[General] once in six month / once in every six month

[Kumar Nadimuthu]

As per the Schedule Q, the frequency of calibration for Total Station instrument is once in six month / once in every six month. However new equipment calibration is valid for one year.

Which one should I use in the underlined part? Should I use definite article 'the' before 'frequency?'

 

[Tdol]

I would use once every six months, or biannually.

 

[GoesStation]

I would use once every six months, or biannually.

Semiannually is more precise. Biannually​ can also mean "once every two years".