Given a string `s` and an integer `k`, return the length of the longest substring of `s` such that the frequency of each character in this substring is greater than or equal to `k`.

Example 1:

    Input: s = "aaabb", k = 3
    Output: 3
    Explanation: The longest substring is "aaa", as 'a' is repeated 3 times.

Example 2:

    Input: s = "ababbc", k = 2
    Output: 5
    Explanation: The longest substring is "ababb", as 'a' is repeated 2 times and 'b' is repeated 3 times.

Constraints:

- `1 <= s.length <= 104`
- `s` consists of only lowercase English letters.
- `1 <= k <= 105`