The dining plan is tied to the number of nights you stay. So, if you are staying 8 nights, but only want to have the dining plan for 4 nights, you'd have to make a room only ressie for four nights, then a package ressie with tickets for four nights, and add the dining on to that. The dining plan is good from check-in through midnight the day of check-out.
Hope I am explaining this well enough... if you do a split stay, you can get dining for just a part of your vacation. If you make one ressie for the entire stay, you will get dining for that entire time.