Is there a guide for how much
planning you should do?
The correct guide is that you plan until you feel that the benefit to be gained from additional planning does not justify the delay or expense of further planning. Many authors propose a general principle of 10% of project effort going into planning (recognizing that planning activities are greatest at the beginning of a project but continue to a much lesser extent throughout the life of the project until closure). For some projects, this can fall as low as 3-5%. In projects that are less well defined (such as high technology development projects), the percentage of effort that goes into planning can go as high as 25% or more.

The wise project manager seeks a balance where the amount of planning is appropriate to the size, risk, and complexity of the project.