when the weed starts to show a white/greyish color on top of the bowl.. put your finger on it and it will smush into dusty ash.. thats when its cashed.
i always punch my bowls (pull the weed through into the bong/ashcatch) but make sure all the green is burnt first!! i pack the weed down while filling the bowl to make sure it wont punch until it is ash and easily pulled through.