Frozen fruit does not have a shelf life in the way a chilled product does. Microbiological deterioration effectively stops at commercial frozen storage temperatures; what continues is physical and chemical change, and its rate depends almost entirely on how stable the storage temperature has been.
What the declared period actually means
A supplier’s stated period, commonly around two years for IQF fruit, is a quality declaration made under an assumption: continuous storage at the declared temperature with minimal fluctuation. It is not a safety date and it is not a guarantee that survives a broken chain. Regulation (EC) No 852/2004 requires that frozen foodstuffs are kept at a temperature that does not compromise safety, and the ATP agreement governs the equipment for international carriage; the commercial quality outcome is a contractual matter on top of that.
What actually degrades
Three things, in rough order of commercial impact.
Ice crystal growth. Every temperature fluctuation partially melts and refreezes the surface water, and the recrystallised ice is coarser than the original. Cell walls rupture, and the fruit releases more juice on thaw. This is the mechanism behind rising drip loss in fruit that has been stored for a long time in a poorly controlled room.
Sublimation and freezer burn. Water migrates from the fruit surface to the coldest surface in the pack, leaving dehydrated, discoloured tissue. Packaging integrity is the main defence, which is why liner specification is not a trivial detail.
Oxidation and colour loss. Slow, but it is what turns a bright red berry dull over a long storage period, particularly where the pack admits oxygen.
The practical reading
Ask two questions rather than one. First, what is the declared period and at what temperature. Second, what temperature record exists for the period the lot has already been in storage. A lot with a long declared period and no storage record is a lot you are buying blind. The cold chain management guide covers logger placement and how to read an arrival record.