How We Design and Plan at DataXu: “Gruhl’s Rule”
Willard Simmons

10% does sound right to me, but maybe in a bit different context. The agile/scrum practice I’m used to is two weeks cycle with one day for sprint planning (so it’s one day out of 10 working days which is 10%). 
The overhead of planning isn’t light (e.g., prioritizing tasks and putting them in the right order, agreeing on hours for each tasks, etc.) so having it more often than once in two weeks seems unfeasible. But at the same time, having it less frequent than once in two weeks wouldn’t have worked because of evershifting (business) requirements and priorities. I was quite surprised when my friends at more traditional software companies (e.g., Microsoft) told me they have much longer dev cycles (which could range from weeks to months), and I assume they would do the planning upfront which could be 10% of the entire time and resources devoted to the project. If that’s the case, it could be they have less external (business) variables that would require adjustments in dev planning and executions.

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