It's not unheard of for someone who's been declared clinically dead to subsequently come back to life. (Let's be clear, by 'clinically dead' we don't mean 'we thought he was dead but it turns out he was just in a coma or sleeping'. We mean verified, flatlining beeeeeeeeeeeeep.) Clinical death is actually medically distinct from what everyone thinks of as death.
But if you're going to come back, you've generally got a window of, at the most, three minutes, at least if you're expecting to go on to make a full recovery. That's how long your brain will usually begin to sustain permanent damage due to ischemic injury, a thing all organs accumulate due to loss of blood flow.
Generally, at least. Every human dies, and seeing as we document as many deaths as we possibly can, you have what amounts to an immense sample size. And in a sample size that large, there will be outliers. So what's the record?
It appears to be an hour and a half, set by Don Piper of Houston, Texas.
Going into his near-death experience, Piper was a minister, having been one since 1984. Five years into his ministry, in 1989, Piper was on his way back from a church conference when his Ford Escort was struck head-on by a tractor-trailer. Clinically, he was killed instantly. Four different EMT's arriving on the scene would verify this.
Meanwhile, from Piper's perspective, he was standing outside the gates of Heaven meeting with dead loved ones. His experience interpreted Heaven as Christian-exclusive. The thing is, though, Piper admits that to be merely his interpretation; he was not told that explicitly by a higher power and in fact went the duration of his experience without a meeting from Jesus. He rather, in his website, uses John 14:6 to back up his claim, "No man comes unto the Father but through me", as well as having only met with other Christians.
In addition, near-death experience researcher Rene Jorgensen notes that Piper's experience is not unique. Jorgensen argues that people with near-death experiences tend to see afterlives compliant with whatever faith they've practiced in life, even if some experiences turn out to contradict one another. Essentially, you see the afterlife you think you're going to see. If you're a Christian, you get sent to a Christian afterlife. If you're Muslim, you get sent to a Muslim afterlife. Jewish, Buddhist, Shinto, Baha'i, Flying Spaghetti Monsterism, Aztec/Hindu Fusionism, the afterlife aims to please. As Jorgensen sees it, there seems to be no one single way to a desirable afterlife. You follow your own path, the other guy will follow his, and perhaps everyone can have their own happy ending.
Back to the side of the road in Texas. A fellow minister, Dick Onerecker, showed up within the hour and, according to him, God told him to start praying, which he promptly did next to Piper's body, which authorities allowed him to remain next to. After his praying, Onerecker began to sing the hymn "What a Friend We Have in Jesus".
Which Piper ended up singing along to. Cue a long, hard recovery, followed by rock-star status in the Christian community, his own ministry, and a book, 90 Minutes In Heaven, followed by Jorgensen's rebuttal and expansion on the subject, Behind 90 Minutes In Heaven.
Piper got 90 minutes. You likely have three at the most.