Many English tests will have “how often…” questions like “How often do you go shopping?” or “How often do you go to the hair salon?”
Personally I go shopping about once a week. I go to the hair salon about once every two months.
When we are talking about a period of time that is singular (a minute, a day, an hour , a week, a month, a year…) we just use a number (once, twice, three times, four times…) and “a/an”
“I shave once a week.”
“I check my Facebook twice an hour.”
When we are talking about a period of time that is plural (five minutes, two days, six weeks etc...) we use a number and “every”
“Babies eat once every three hours.”
“We start a new project at work once every six weeks.”
Sometimes you can answer a frequency question without a set time, but rather a condition. For this it is common to use “When” or “Whenever”
“I go shopping when I run out of food.”
“I go to the hair salon whenever my roots start to show.” (here they are talking about the roots of their hair, that show the natural color).