Ehh

new mytimer = 0, somecounter = 0;

public OnPlayerStateChange(playerid, newstate, oldstate)
{
    if(oldstate == PLAYER_STATE_DRIVER)
    {
        // If a player has left its vehicle successfully
        // Add your additional check here...
        somecounter = 0;
        if(mytimer != 0) KillTimer(mytimer); // If the timer was not being killed, gets killed properly...
        mytimer = SetTimerEx("blahblah", 1000, true, "i", playerid); // Sets a repeating timer, and stores the timer ID into some global variable
    }
    //...
    if(newstate == PLAYER_STATE_DRIVER)
    {
        // If a player has entered a vehicle successfully
        // Add your additional checks here
        if(mytimer != 0) KillTimer(mytimer); // Kills the timer
        mytimer = 0; // Resets value
    }
}

forward blahblah(playerid);
public blahblah(playerid) // Our callback
{
    // Add your code here
    somecounter++;
    // Add your checks here, and if the counter has exceeded...
    if(mytimer != 0) KillTimer(mytimer); // Kill the timer to prevent calling this callback once more accidently
    mytimer = 0; // Resets the value, of course
    somecounter = 0; // Resets the counter
}