I believe you have a maximum of 18 days in which to use the 14 day ticket.
In our recent 19 day trip (7 on-site, 12 off-site) we managed to do 9 Disney park days, but then we did also have days at Universal, SeaWorld & Busch Gardens, plus a few days off to relax, shop and explore, so not using our tickets to the max wasn't a big concern of ours.