Viewable refresh can be configured in HTL BID for each slot individually:
Configuration
Navigate to “Ad Configuration” > “Slot Setup” and expand a Slot
Scroll to the end and click on any attributes or the arrow on the far right to expand the Slot Attributes section, locate the “Refresh” checkbox
The two fields will be pre-populated with default values that can be edited:
Interval (s) - The number of seconds between each viewable ad refresh. The ad must be continuously in view for the set timeframe
Max Refreshes - Maximum number of refreshes allowed per slot