Have a Rules condition that will take the current time minus last comment posted time Overall goal:  If a node has not been commented on in a given time frame, send an email.   

How it could possibly be done:  Have Rules look at the last time a comment was posted when cron runs.  Then do a data comparison of that unit (current time - last comment posted time) to see if its greater than the time frame assigned to the node (ex: hours).  If so, send email.  

Is this approach correct?  I am struggling to get the Rule to work if it is.
Overall goal:  If a node has not been commented on in an in a given time frame, send an email.


Here is a bare bones solution. On paper this should work.

Basically, we use sticky as the tell tell sign if a new comment has been added (if it's not sticky, no one has commented). Once the e-mail gets sent, we set node to unsticky. 

1) Create a rule after saving new comment. 


Add Action: Under Node, Make Content Sticky >> comment:node



  Then it's very similar to my answer to your question Rules Conditions
  being evaluated after schedule time
  elapsed?
  
  Create a New Component
  
  1) Add Compotent of Rule.
  
  2) For Data Type: select Node. Name, Label and Machine name use
  whatever you want.
  
  3) Add condition of Content is of type
  
  
  For Data Selector: put your Node Data Type machine name. Ex: if you gave the node data type a machine name of test you put test
  Select your content type
  


4) Download and Install Conditional Rules Module

Then for action, click on Add IF >> DATA COMPARISON >> test:sticky >> False

Then in IF: Data Comparison row click Add Action >> Send Mail

Then in Conditional row (1st row white) you will now see an Else button, click on it. 

Then in Else click on Add Action >> Make Content Unsticky >> test




  Now Create a new Rule
  
  1) For action use after content is created
  
  2) Leave the condition empty (no need for condition here)
  
  3) for Action add the scheduled component evaluation and select the
  component you just created.
  
  
  use node as your data selector