A lot of good advice here Steve. As said, ingredients do have a bearing on storage time. However, unless you know exactly what the ingredients are (unlikely, unless it's your own mix) AND are knowledgeable enough to understand which items effect the outcome, that information doesn't help a lot.
In that case, my thoughts when carping were these.
Don't try to keep the average boilie for much more than a year, and only that long if the bag has remained sealed as received from the supplier.
Check through the bag for freezer burn, and reject bags (at least as hookers) where the majority of baits are affected.
When thawing the baits in preparation for use, leave the bag sealed, giving it a good shake occasionally, so that any liquids forced out by freezing (often containing flavours etc.) are evenly reabsorbed.
When the bag is finally opened, allow the baits to stand for a while, then check that they still look, smell and break between your fingers exactly as they did when fresh.
If all those conditions are met, you won't go far wrong. It is a fact that boilies older than that, or not up to that standard may (and often do) still catch fish, but the results are not nearly as consistent in my experience....and that means those nagging doubts will start to sap your confidence in very short order if bites are not quickly forthcoming. NOT a recipe for a relaxing, fruitful days fishing
Cheers, Dave