time_t reftime = 0;
bool ShowHealth = 1;


void TimerThink()
{
   if(reftime - time(0) > 5)
   {
      ShowHealth = ShowHealth ? 0 : 1;
      reftime = time(0);
   }
}

//draw code:

TimerThink(); //not a good place for this tbh, but whatever

if(ShowHealth)
{
    //draw health
}

#include "time.h"