How do I change the Time Warning? (Dasher)

Click on the Settings Icon   from the top toolbar > A window opens with Tabs above it. Click on 'Playing'
Or menu above: Edit > Settings > Playing Tab  
To turn the warning on check 'Low-on-time Warning'.  Unchecking will turn it off.
You can also set the amount of seconds you want to get a time warning.  Example: 30 will give a warning when 30 seconds left.
   
When done click on OK.