Quickest turnabout I could manage with a short run and a dive. It's better to get shrapnel in your legs and back than your face and torso. Even the fuses on them aren't entirely reliable. If you're told it has a five second fuse, then it has a three second fuse. If I had somewhere to dive, then I'd go there, otherwise, the turn, duck, and cover. Only an idiot or a hero would throw it back or jump on it, and I'm neither.
This thread did give me a rather amusing mental image of two guys having a grenade landing between them. Both dive to grab it and throw it back and end up knocking their helmets together and knocking each other down. Ultimate "oh shit, I screwed up" moment I can imagine.