The answer is "sufficient". You need to spend the time to define the work, create a schedule, estimate the costs and set up the project management processes. If your project is small, this should not take much time. If your project is large the planning may take a log time. In other words, planning is scalable based on the size of the project.
Spending time on good planning ends up taking much less time and effort than having to correct the problems while the project is underway. We all know this to be the case. We just need to practice this on our projects.
Before the project work begins, the project manager must make sure that the work is properly understood and agreed to by the project sponsor and key stakeholders. The larger the project, the more important it is that this information be defined formally and explicitly. When you think about it, many project problems can be traced to problems in planning. These include
Poor estimates based on not understanding the totality of the work.
Lack of scope change management because scope was not properly defined to begin with.
Issues occurring because of poor risk management.
Missing work because the schedule is not thought out.
Not understanding all the stakeholders involved.
It should not be surprising, then, that the best way to avoid this problem is to do a good job of planning the project up-front.