How do I find the last date and time when my iPad was backed up?

Go to Settings. Tap on your iCloud ID at the top of the page. Then iCloud>Devices Backups>iCloud Backup. You'll see "Last successful backup" just under "Back Up Now".

If you have that turned on and you connect your iPad to WiFi while charging it overnight, it should back up every day.

Adding to IdrisSeabright’s response …

That response is exactly correct assuming you back up to iCloud. If backing up to a computer, once the iPad is attached, examine “Manage Backups” in the iTunes or Finder window.

UPDATE - Never mind, this must be irrelevant as we’re in an “iCloud on iOS” discussion area.

It's still useful information!