Item? Im assuming you mean purchased dinos? :)
They are hardcoded to 15mins, you will just have to use them within that timeframe.
What do you need extended time for?

im making events. im hiding boxes around the map and let the peaople search the boxes for winning the stuff in the box. 15 minutes are not long enough for this funny event.

maybe you have an idea?

thx mate

How about making a Pack available for anyone in the WINNER category or something of the like, purchasable only by the ones you deem to be the winner? Then the spoiling time is plenty and you can hide anything without worrying about spoiling time :)