Quote from PsiQss
From what I understood from the wiki, they explode immediatly after falling near the player. Never saw an actual creeper explode after taking falling damage though.
When a creeper takes fall damage, it will prime itself to detonate immediately, but won't actually explode unless within range of the player.
An easy way to see this is to shoot a creeper off of a cliff or a ledge, you'll notice that when it hits the ground it will flash white in addition to red and grow like its about to kaboom.
Another easy way to observe this is hang out in a ravine and watch them fall. Though usually when this happens, it's because they fall on top of you and explode on you. Haha.
It's also important to note that I play on Hardcore mode, which means the game is locked on hard difficulty. Creepers love to explode on hard even when you're outside their explosions damage radius. And a point blank creeper explosion will kill you with full iron armor on.
2/3's of the time, when I die on Hardcore, its because a creeper fell on me in a ravine or cave.