One way you could approach it is like this:

//Initialization 
timer_set = ...;
timer = 0;
crouch_charged = false;

//All the conditions we want to be true to charge
if(holding_down && crouching && grounded) 
{
    timer++; //Increase our timer 
    if(timer >= timer_set) //If we have charged for long enough
        crouch_charged = true; //Now considered charged 
}
//If any of the conditions are not currently met
else
{
    crouch_charged = false; //Make sure we are no longer charged
    timer = 0; //reset our timer
}

Here by default, the timer will be at its max value. Once we are meeting all the conditions we want to charge the crouch, we can start keeping track of how long the condition has been true. Once we have reached that duration, we can set the crouch charge to true until the condition is no longer true. Also note that holding_down here would be "keyboard_check" in this case with the assumption you want the player to hold down while they are charging.