Once a week is "isshuukan ni ikkai". isshuukan means "one week" ni is like "in" ikkai means "once"

The most basic way has been already explained by nekochan, so let me give you some of other alternatives. 週に一回(shuuni ikkai) 週に一度(suuni ichido) 一週間に（isshuukan ni）can be shortened to 週に（shuu ni） especially in spoken Japanese.