It's already possible to revive people who've been clinically dead for several minutes. And we know that Trek medical science is even more advanced. It tends to be forgotten that the first season of TNG portrayed it as advanced enough to reverse recent death in multiple cases, such as Yareena in "Code of Honor" and the cryosleep subjects in "The Neutral Zone." This was largely overlooked in later seasons, though.
Some medical practitioners have recently been talking about potential revival hours after death, at least in the right conditions. I don't know whether they're being overly zealous or not, but some of them seem to think we're getting close.
I'm surprised people in the Trek 'verse don't enter stasis on their death beds, actually, and wait to be revived when the immortality serum/miracle cure-all finally gets made.