1. From the menu choose Central dropdown => Shop wide Discounts 

2. After you have setup your discounts, tick the box "Use time limit" as shown in the screenshot below. 

3. It will enable the Set Time Limit button, so click on that.

4. Now you will see the time limit discount window:

- Set the Start and End dates properly 

- You have the option to enable "Repeat after" a number of days, weeks or months.

So for example you can repeat the discount every 6 months

You also have an option of How often.

For example I want this to repeat every 6 months 2 times only then i would set it up like this