I'm always wondering when to change the filter for healthiest water.
The filtes used to have a mechanical dial based on number of fills and
then they downgraded to a digital one based on time. But it seems
(since my taste is unreliable) a better way wol be to judge by rate. I
am thinking that as long as one of the columns comes out in a
continuos stream it is good.
Obviosly the real tests would involve lab use over time.

I've wondered that, too.

I take no notice of the dial, which I suspect is just a marketing
device for their filters - which are expensive.

Taste-wise, I have never been able to detect any deterioration in the water.

Of course, if I die from water poisoning then the Coroner will have no
problems in determining the cause ...

But let's face it, water straight from the tap where I live (the UK)
is very good anyway.

A further thought ... The dial seems to be time related whereas I
would have thought that the filter would have been principally
impaired by the volume of water processed.

I can tell you that one time we failed to change the filter in a
timely fashion and the resultant water was horrible.  It seemed to be
dumping all the collected junk back into the water.

Activated Charcoal, the "black stuff" inside of your Brita filter has
a surface area of anywhere from 150-15,000 m^2 of surface area per
gram.  Theoretically, if your water is pretty clean, A LONG TIME would
be your answer.  The filter will predominately remove Organic
compounds that are greater than 3 carbons in length from systems.  It
will also remove Inorganics, such as salt, and large metallic chelates
as well.

Theoretically, you could also recharge your Brita filter by heating
the "black stuff" to 800C.  However, this would only burn the volatile
organics, and destroy the Inorganics that decompose at 800C.  However,
I haven't figured out how to pull the thing apart without breaking it.
Molecules are Adsorbed onto the surface of Activated charcoal, not
absorbed by the charcoal. Analogy: Water is absorbed into sponges,
dirt is adsorbed onto a sponge when wiping with it, and that's not
truly what is happening, but it is a simple analogy I heard once to
explain to our salespeople.

Mainly,  keep the inside of your pitcher clean. That way, you can
visually inspect the bottom of it right below where the filter expels
into the pitcher.  I live in an area that has a lot of iron in the
water.  When I start to see a reddish deposit below the filter, I know
it's time to spend $5 for a littler cleaner water.

That's a lot of great detail familiaroldsetc!
I've seen a lot of references on the web to this but I'm still
wrapping my head around the 15000sq. meters.
What is the minimum weight of 15,000 square meters?

Also what do you think of the slowdown in filtration rate after a few months?

I spent the last hour reviewing basic chemistry so I may have gone
wrong somewhere...

the 15,000 sq. meters we will give the minimum volume of 1 atom thickness.

15,000 cubic. meters/ volume of zinc=number of molecules
number of molecules/avagado's number=weight in grams*atomic number.

15,000 cubic. meters/ volume of zinc=number of molecules
number of molecules/6x10^23=weight in grams*atomic number.

Power of activated carbon: 1 gram=15,000meters
Zinc density=71.4g/cm cubed

15000meter*1,ooo,ooocm/meter=number of centimeters
7.14 g/cm cubed*number of cubic centimeters= weight in grams

15000meter*1,ooo,ooocm/meter=15*10^9  cubic centimeters
7.14 g/cm cubed*15*10^9= 1.05*10^10 grams of zinc

let's assume a brita filter weighs only 1/10 of a kg
Therefore a brita filter can absorb 1.05*9 kgs or 1 million kgs or 1100 tons

The weight of the empire state building is 365000 tons therefore it
would take about 340 Brita filters to capture enough zinc of
equivalent weight to the Empire State Building.