One way to know how long your next work project will take is to identify the critical path. The path consists of all the critical tasks -- those you absolutely cannot postpone without slowing down the project. To calculate the path, you add up the time it will take to complete the critical tasks. That gives you the absolute minimum time you'll need to finish the project.
A non-critical task has slack time. If it's delayed by, say, a couple of days or a week -- the amount of slack depends on the task -- it won't push your project past the deadline. Critical tasks have no slack: any delay finishing the task delays the project. A task with flexibility in start and finish times probably is non-critical. Critical tasks have no flexibility: They start and end on specific dates or after a set period of time. They're sequential, with one critical task starting when the previous one ends. A delay in one task delays all the following tasks.
Suppose you go over the project and find it has four critical tasks. If the first critical task will take a week, record that. Do the same for the other critical tasks. You can use assigned deadlines if you have them, or you can rely on your project-management experience to gauge the times. Add the tasks together and you have the duration of the critical path.
When you add up the times, you may find that the critical path is longer than you've been given to work on the project. If you can't get an extension, you have to change the critical path. Putting more staff on a task or authorizing overtime, for example, would allow you to get tasks done faster.