I think that your PBMCs are ok there for that time, anyway you should think about a system for test the viability of your stock solutions after several months of storage.
Look that http://www.google.de/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CDcQFjAB&url=http%3A%2F%2Fwww.cryocatalog.ru%2Finfo%2Ffreezingprotocol.pdf&ei=8jI_UeXCAcGYtAaF1YDYCQ&usg=AFQjCNHCxPYXGrs8Kw57LiLxNMPXI6TDsQ&sig2=AoVWG2f8MreeHs7NQH1JZA&bvm=bv.43287494,d.Yms&cad=rja
I have storaged Human PBMCs in these contitions for more than 6 months. I had no problem. I checked both DNA (with STRs) and RNA with qPCR, pre and post storaging and everything was fine.
Hi, I also have stored human and rabbit cells in -80°c for about 6 months and I was able to recover the cells with viability. Always perform a viability test after unfreezing your cells.