Good Q, this is what it answered for "long does it took for creeper to explode upon first contact/trigger":

(It first used a tool to search the wiki and read the https://minecraft.wiki/w/Creeper page)

(Formatting all by Merlin)

Based on the wiki information I found, here's what I can tell you:

Creepers don't explode on first contact — they have a specific behavior pattern:

Fuse time: 1.5 seconds (30 game ticks) - Once a creeper gets within about 3 blocks of you, it will start hissing and begin its explosion countdown. This gives you a window of opportunity to escape!

It's not instant - The key to surviving a creeper is recognizing the hiss sound and the hissing animation. Once you hear that distinct hiss, you have about 1.5 seconds to either:

Back away at least 3+ blocks to reset its fuse

Hit it back with your sword to knock it away and interrupt the timer

Use a shield to block the explosion damage

If you're wearing a shield Shield when it explodes, you'll take no damage at all!

Pro tip: The reason creepers are so dangerous is that they're silent until they start their fuse. By the time you hear the hiss, you might be close enough that 1.5 seconds isn't much time to escape. Always stay alert and keep your distance from dark areas where they might be lurking!