Quote Originally Posted by eranmaayan View Post
This is a good answer but it reveals the weakness of GTD. which gives you you a good method/intuition of what you should do now in the light what you have on your plate now but does NOT give you any method of PLANNING multiple projects. That means you have no ability to know when will your new project end or how will it push your current projects if it is high priority. True , your will have better feeling but you can not base it . I just wanted to verify this. I will be glad to be corrected.
What Brent said. See also Chapter 3 of the GTD book, on planning, in which DA discusses some of the limits of his back-of-the-envelope planning model.

In my experience, the first step in deciding when a project will end is to develop a good estimate of how long it will take. That estimate must come from your own domain-specific knowledge. Only then can you even begin to discuss workload management and planning and scheduling for multiple projects. Seems fundamental, but is actually remarkably difficult.

Katherine