Enum describing time window over which we want to reach Target Frequency.

[Proto definition ![GitHub](https://developers.google.com/site-assets/logo-github.svg)](https://github.com/googleapis/googleapis/blob/master/google/ads/googleads/v26/enums/target_frequency_time_unit.proto)

| Enums ||
|---|---|
| ## `MONTHLY` | Optimize bidding to reach Target Frequency in a month. |
| ## `UNKNOWN` | Used for return value only. Represents value unknown in this version. |
| ## `UNSPECIFIED` | Not specified. |
| ## `WEEKLY` | Optimize bidding to reach Target Frequency in a week. |