The official word I just got from Buckman Laboratories that make the PolyQuat sold in the U.S. is as follows:

PolyQuat is a biocide that is registered with the USEPA; and, as such, it will have a specific guaranteed shelf life. The shelf life (which may or may not be printed on the label) is one year. In general, registered biocides are assumed to have a one-year shelf life unless the label states otherwise. While the compound itself is very stable and is not expected to decompose upon storage, we cannot guarantee it beyond the stated one-year shelf life (due to legal and regulatory considerations).

In other words, it is "guaranteed" to last one year, but it's a stable compound so would presumably last longer. How long -- well, who knows. And be sure to check the label to see if it states otherwise (i.e. a shorter shelf-life).

Richard