Typically you have 1hr to complete a survey and 48hrs to complete an app download task. After this period, tasks will be marked as expired/abandoned and they will no longer be available.
When you start a task you should try to complete it in one go. This is because you only have one chance to complete it. If you leave the task, it will be considered abandoned and you won't be able to access it again.
Remember you only get paid when a task is fully completed.