In most cases, your vacation response is only sent to people the first time they message you.
Someone may see your vacation response more than once. If the same person contacts you again after four days and your vacation reply is still on, they'll see your vacation response again.
Note: Messages sent to your spam folder and messages addressed to a mailing list you subscribe to won't get your vacation response.
To set up your automatic Gmail Out of Office "Vacation Responder"
Click on the Cog in the upper right corner of Gmail.
Click on See All Settings.
Scroll to the very bottom of the General Settings tab that opened.
Check "Vacation responder on"
Add the first and last date of the time you will be out of the office. It will start at 12:01a on the First Day and end at midnight of the Last Day.
Type in the Subject for your automatically generated emails
Type a brief message about how the contact can get assistance in your absence.
Check the bottom two boxes as appropriate. Checking the boxes will eliminate sending "Out of Office" email to automatic newsletter and email lists that you receive, but also eliminates people that you rely on Gmail's autocomplete email address feature to save email addresses and only send to those that are actually listed in your contacts.