You can only be assigned ready reserve for half the available reserve days in a month. They are also limited to giving you no more than 3 in a stretch. So if you have 6 days of reserve you will get no more than 3 ready reserves, and if you had 22 days of reserve you can not get more than 11 in the whole month.
Also on the last day of reserve you are released at 1800 no matter what. Only way this would not happen is if you were assigned a trip that ended after 1800.
So you should not have an issue making the last flight to most cities if you are not used.