That is a good question. It obviously does not remain effective, or we would not need to use it at every water change. And Seachem do state that the detoxification of ammonia/nitrite/nitrate by Prime is temporary, about 24-36 hours after which these three if still present revert back to the toxic form, so this is some sort of binding that is not permanent.
Using more than absolutely necessary is however harmful to fish.
First, like any additive, conditioner increases the TDS (total dissolved solids) and as these increase, the fish are negatively impacted (soft water fish more than harder water species). The TDS remain until we remove them via the next water change.
Second, like all additives, conditioner is pulled inside the fish via osmosis through every cell, and at the gills. It enters the bloodstream and is carried to the internal organs. Manufacturers saying this is not harmful is not being sensible; if harmful means the fish do not die within seconds, fine, but I have a much broader interpretation of "harmful." We need to use conditioner, but it should not be used at a level beyond what is essential. This is one of the main reasons I use and only recommend API's Tap Water Conditioner; it is the most concentrated product (1 drop per gallon, 1 ml treats 20 gallons) so less is used and therefore less gets inside the fish. It also contains no unnecessary additives to cause more trouble.
itll be tough to cycle with cories, there may be some losses, but keep up on it
