The short answer: a lot earlier than you think.

The longer answer: there is a tendency in many coaches (including me), to work on a playbook endlessly. I am guilty of it. I create playbooks that are way too complicated. They contain so many details that my coaches and I will never be able to teach to our athletes.

"A complex system that works is invariably found to have evolved from a simple system that worked. The inverse proposition also appears to be true: A complex system designed from scratch never works and cannot be made to work. You have to start over, beginning with a working simple system." -- John Gall, author of Systemantics: How Systems Work and Especially How They Fail

You might love to develop playbooks in great detail (like I do), but is it worth your time? Spend more hours if you develop playbooks as an intellectual exercise. But stop early if you would like to get results from it.

I recommend developing the very basics and one or two plays, and then go out on the field with your team and try to teach it. The insights you will gain will be much more helpful than another two hours spent. Two hours spent on a dozen more plays, and very neat drawings. 

A useful question to that end: 

What's one decision you can make today, in your tactical model,

that will save you from having to reverse ten other decisions in the future? 

Make significant choices, i.e. ones that matter for a long time. Simple but not easy.