This is a feature that works in conjunction with the "minimum load" feature. If you have "minimum load" set to a number other than zero, the convoy will wait until the vehicle is loaded to that percentage. For example, if you set "minimum load" to 100%, the vehicle will not depart from a stop until it is completely full.
However, if you set the "month wait time" (if you have downloaded the English translation texts - included in the Simutrans-Experimental-complete .zip file or in the configuration files .zip file - this will appear as "Max. wait for load"), this will set a maximum amount of time that the convoy will wait. For example, if it is set at 1/2, it will wait for half a (game) month for a full load. If it gets a full load within that time, it will leave immediately. If, after the end of half a month, it has still not got a full load, it will leave immediately anyway.
I hope that this helps.
