A person shall be imprisoned for no less than 90 days if convicted for a second DUI within how long?

The answer varies depending on the state and the severity of the offense but it is mostly 10 years.

In CA, if any person is convicted of a violation and the offense occurred within ten (10) years of a separate violation which resulted in a conviction, that person shall be punished by imprisonment in the county jail for not less than 90 days nor more than one year and by a fine of not less than three hundred ninety dollars ($390) nor more than one thousand dollars ($1,000).