It’s really difficult to fix a project time. You have to think your work ability, risk management, QA and finally your experience will help you to assume a project completion time. I have seen when we ask to the developers how much time you will take to complete it, they reply 1 day, which is the best they can do…. So easily I understood that actually they are going to take 2 and a half time. But clients don’t want to listen that they are always in hurry. So we had to plan that this work will take 1 and half day.
Finally what happen, developer couldn’t finish it within a day, and they took another extra hour on that night and half of next day to complete the task. So, we don’t have time to check properly but had to submit it to the client. Client found some bugs, so return it and developer and QA took another day to make it perfect. And the project took totally 2 and half day but the client paid only for 1 and half day as they planned.
So, you have to keep in mind all of these factor before planning for project timing.