How much max time you invest in writing marketing copy?

I personally took maybe A month or two writing a three-pager of marketing ideas, communications, value props, strategies etc. This was well worth it as now any type of initial marketing we do somewhat feels seamless as we invest a lot of time upfront thinking about how to frame our startup, how to communicate our value props, and what we want our brand identity to be. This has made creating marketing material and communicating our vision with external contractors/nano influencers very effortless ahead of our launch of the first half of our marketplace come July 17th for sign-ups

You should honestly be testing value proposition copy before pulling the trigger on any product stuff. So, it follows, that the copy writing cycle lasts for the entire sprint.