Although it doesn't get a lot of importance, testing is arguably the most important phase in any software development project. Certainly testing is —the only— way to know whether your project or product is destined for success or doomed to failure before it goes live.

For most projects, I try to stick with the 1/3 rule:  spend (approximately) 1/3 of your time on planning, 1/3 on coding, and 1/3 on testing.  Many programmers expect to spend the majority of the project timeline writing code, and believe that good code will drive the project.  However, I have found that most worthwhile final products are created from development timelines that have actual coding as a minority.  This 1/3 flow can often take iterations for different portions of a project, where you plan, code, and test small portions and move on to the next section.  But you will often find dedicating time to plan properly will cut down on actual coding, and proper testing will save you time in the long run.