At what frequency should a brand launch on facebook and youtube?

It's a very vague question. It's all depend on brand itself. Some brands require more frequency than other. First you need to understand your audience where they are coming from, what platform they are using, what's the engagement rate, what time they are most engaged, and how they are consuming your content. Once you have basic analysis then promote the brand and do a test run. For youtube, the best cast is to start with small amount of video PPC and do a test run. You will get a clear picture within two weeks.