Drowning is a process, not a single event. That is, drowning takes a
rather extended time period, rather than an instantaneous action.
At
its most basic level, drowning is suffocation, depriving the body of
oxygen cells need to function. From a medical standpoint, the most
critical cells are brain cells; other cells can die, but if enough brain
cells die, then the body dies, no matter how much of the rest of the
body's remains alive.
For this discussion, let's assume that
drowning starts at the moment the person takes their last breath of air,
NOT when they fall in the water. After that moment, the bloodstream
does not get any new oxygen. So, as it circulates, cells remove the last
of the existing oxygen from the blood; this process takes a handful of
seconds (perhaps as many as a dozen). After the bloodstream's oxygen is
depleted, cells switch from aerobic (with oxygen) to anaerobic (without
oxygen) chemical processes to get their needed energy. Depending on the
physical fitness of the individual, a person can burn anaerobically for
up a couple of minutes at full speed; think about the time that you can
hold your breath - that's the time you are burning anaerobically at full
speed.
After this time, the body recognizes that it is in
immediate danger, and begins to shut down various functions, in an
attempt to conserve any stored energy. In the brain, this shutdown
quickly leads to loss of consciousness. The body continues to slow down
as much as it can, while still burning reserves anaerobically to provide
energy. Eventually, the body runs out of reserves, and cells die. When
enough brain cells die, the person is dead.
Restoring oxygen
to the person at any stage above can revived them, though, if the person
has been without air for an extended time, brain cells may have died,
which is permanent. That is, recovery may be possible, but brain damage
is permanent.
Assuming a normally fit person, otherwise
conscious and uninjured, who is drowning in warm (21C/70F) water, I
would estimate 2-3 minutes before full loss of consciousness, and 5-10
minutes before brain damage, and 15 minutes before irrecoverable death.
Note
that there is a recently discovered reflex which helps humans when
drowning - it is called the Mammalian Diving Reflex.
When cold (below
21C/70F) water hits the face, the body immediately begins the
"shutdown to conserve" process above. This only works if the face is
underwater. The colder the water, the faster the body shuts down. Also,
the younger the person, the faster the reflex takes action. There are
documented cases of children falling through ice-covered ponds who have
survived over 30 minutes without brain damage.