Monthly change out is what Periodic Products (the mfg) recommends, but I suspect that recommendation has more to do with their sales goals, than with necessity.

From their own data, it appears that color intensity indicates when the product is 'consumed'. In the pictures above, the CuLator looks to me like it still has a LOT of use left.

A cautious approach might be to put a new CuLator in beside a month old one. That way, if the older one really is failing, the pool is protected. But if it is NOT failing you'd be able to determine that, as the older unit continues to darken in color.