Step 1 - Login to Usermin, and you will be presented with a screen like the below. 

Step 2 - Select Automatic Reply 

Step 3 - Complete automatic reply as per the screen below, setting your message and dates accordingly. 

Step 4 - Hit SAVE

Step 5 - Don't forget to logout of UserMin

This will automatically reply to your email's during the window defined with the message you defined. You can also do this in Email Filters, but that is a more advanced route to achieve this.