The amount of time you have to complete a task varies from project to project. The processing time always starts once you start the job. Some do have a time limit, others are available for an unlimited amount of time.
You can find the time limit and further information in the job overview:
A countdown timer also displays the “Remaining Time” on the work page. It's either at the top of the page when you're working in your browser or at the bottom of the screen when you're working in the clickworker app.
You must complete the job before the time runs out, i.e. reaches zero. If this happens the job will be canceled and will be lost for you!