MattH
I recommended osmio to a friend of mine who lives in Swanscombe their tap water is pretty hard, filters last him 5-6months, I think he can get another month or two with resetting the unit but I told him to try hard water filters next time, they are £5 or so more expensive but might last longer. They are really happy with osmio.
We live in a medium hard water area and our filters last more than a year. We are also really happy with it, been 2.5 years. Only time we needed kettle was to fill a hot water bottle:)