From the Check Time menu:
Add
This button opens the dialogue box that will allow you to set a Simple Check Time for MailRules to check your mail server for incoming messages that meet any Inbound Rule criteria.

You have two methods of setting the time, by Specific Instance or by Frequency:
Specific: A specific instance will allow you to specify a day of the week or 'everyday', and the specific time of tat day that MailRules will check for messages, e.g. Monday 8:00 or Everyday 8:00
Frequency: A frequency time setting, allows you to specify a day of the week or everyday, and the interval between checks, for example "every 15 minutes on Monday", or "every 30 minutes, everyday".
Edit
To edit a Simple Check Time, select it from the list and click the Edit button. You will see the same dialog box used to add a Check Time.
Delete
To remove a Check Time select it from the list and click the Delete button.