Project planning is widely thought to be an important contributor to project success. However, there is a little research to affirm its impact and give guidance as to how much effort should be spent on planning to achieve best results. We aim to rectify this omission. Data was collected on 1386 projects from 859 respondents via a global survey. A significant relationship was found between the quality of the planning deliverables and success. Detailed analysis of the data collected revealed an inverted-U relationship between the percentage of effort spent on planning and project success. After correcting for key moderator effects, a significant relationship with an R² of 0.15 was revealed. Further analysis showed that the fraction of planning effort that maximized the project success was 25% of project effort. This was substantially more than the 15% mean value reported by respondents. The greatest impact was found to be on the broad success measures with a lesser effect on project efficiency: time; budget; and scope. The inverted-U relationship between effort spent on planning and project success indicates that projects can spend too much time in planning, as well as too little. But we found that projects are spending less time in planning than the optimum to achieve best results.