The length of time that a cut unfixed slide can be stored in the -80 depends on the antigen being examined. I usually try to perform all of the required staining within 3 weeks after cutting and the results have been consistent. However, I examine the usually microglia markers (CD11b, Iba-1, CD68, MHC II etc), astrocytes (GFAP) and myelin (MBP, Nogo, NG2 etc). Your antigen and antibody might be more sensitive to the drying artefact that is found in the -80. Using one of the machines that can vacuum pack slides might be a good idea.