[seaman]

bool fall;
float timeStartFall;
public limit = 5;
...
if(!isGround) 
{
   fall = true;
   timeStartFall = Time.time;
}
if(isGround && fall)
{
   float deltaTime = Time.time - timeStartFall;
   fall = false;
   if (deltaTime > limit) Char.GetHP(deltaTime);
}