Yes, you can freeze it. I've purchased boxes of "misc. partial bags of LME" and frozen them without any problem. How long? Used the last bag about two years after the purchase. I strongly suggest you re-bag in usable sizes. One of my bags was 8.75 pounds of MO, which basically limited me to a steeped IPA.
Remember one unassailable statistic, as explained by the late, great George Carlin: "Just think of how stupid the average person is, and then realize half of them are even stupider!"
"I would like to die on Mars, just not on impact." Elon Musk