When airborne a regular internet cell connection is problematic at best and quite frankly one shouldn’t depend on having it at all especially with T-storms nearby. ADS-B receiver is a better option but that is a completely passive solution: the app only displays what it receives and can’t request anything on its own. On top of that NEXRAD images, regardless of the source, are transmitted in tiles to save bandwidth (not the whole country at once). Given that you might be looking at, say, 4 different NEXRAD tiles stitched together but only 3 of them are updated and the fourth one isn’t for some reason. The DroidEFB app displays NEXRAD age as the age of the oldest tile currently displayed on the screen so you might see the NEXRAD image changing (some tiles are updated) but not the age (at least one tile didn’t update).
Bottom line is that while it might be possible to force a more frequent updates if you have a reliable internet connection that’s typically not the case when you’re airborne. The next best option, ADS-B, is completely out of control of any single vendor and the app can only display what it receives.
On the ground, with stable internet connection, DroidEFB usually updates NEXRAD within 10-15 minutes.