Researchers have run a simple experiment to see what is the best deadline for actually achieving something. 

A team including experts from Macquarie University mailed out surveys and asked people to complete them in one week, one month or with no deadline.

They got most responses without a deadline, but the one week deadline was still better than the one month deadline.

The authors say this is evidence that specifying a longer deadline as opposed to a short deadline or no deadline at all, removes the urgency to act, so people put off the task and those who are inattentive might forget to complete the task entirely.

The full study is accessible here.