раз в шесть месяцев
• The governor should be tested at least every six months.
• Under normal operating conditions, the instrument should be lubricated at six-monthly intervals.
Русско-английский научно-технический словарь• The governor should be tested at least every six months.
• Under normal operating conditions, the instrument should be lubricated at six-monthly intervals.
Русско-английский научно-технический словарь