By default, LMDE uses Mint's own copies of the Testing repository, called Incoming or Latest. These aren't updated as frequently as Debian's Testing. Currently there are several months between updates (a.k.a. update packs). The idea behind this 'freeze' is that it allows the Mint developers to check the packages arriving in Testing (more) thoroughly before they're offered to LMDE users.
If you don't like the idea of having to deal with 1300+ packages in a pack, once every two to eight months, you can change the list with software sources so it doesn't point at the LMDE repositories, but at Debian's own. That means you'll get updates quite frequently, but it also increases the risk of some update borking your system (after all, you are 'Testing'...). This thread
is used to help people along if they want to take that risk.