Not sure about -80 degrees, but we have stored PBMCs at 150 degrees for over 2 years and they remained viable after thawing and culturing. The most critical step, from our experience , is the initial freezing steps. A gradual freezing step of 1 degrees/min is most suitable and you can ensure this by using "Mr Frosty" freezing container.
Okey's response was apt. The process of freezing largely determine the keeping quality of the PBMC. I know they can keep well also at -80 degrees but for how long, I may not have clarity. However, I have thawed ASF-infected PBMC after several years of storage and still got viable virus but have to change the PBMC as its a primary cell line for ASF virus isolation.