What is the polling interval if I leave it blank?

The same as your setting under Settings (upper right corner of the web portal when you are logged in as an administrator), Polling Settings (right side, midway down), "Default Node Statistics Poll Interval" (which is 300 seconds (5 min) by default.

If you will pardon the interjection, adatole, I believe that if you leave the polling interval blank, the data will not be collected. I have found that UnDP data resources on the web will not populate at all unless that value is populated and at least one poll has taken place.

If I am wrong about this, please point me to the documentation for future reference. Thank you.

Every one of the UnDP pollers I have running right now (~50) have blank polling interval values. Trust me, they're pulling data!

If you click the "more about polling interval" link next to that field, you are taken to the web page that explains what I said - if you leave it blank, it uses your default node (or interface) statistics polling interval.

is right:

UnDPs without specific poll interval use statistic poll interval from node/interface which are assigned to.

But the default statistics poll intervals are 10 minutes for nodes and 9 minutes for interfaces.