The estimated delivery date for a reward is the date you expect to deliver that reward to backers. If you're offering more than one thing in a single reward tier, set your estimated delivery date to when you expect everything in the reward tier to be delivered.
If you're not sure what the estimated delivery date is for a reward, take some time out to create a timeline for your project so that you have a good sense of when you'll complete it. Choose a delivery date that you feel confident about and will be working towards.