For how long can I count on the effect of Campden tablets?

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Jul 4, 2023
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St Albans, UK
Hey everyone! I'm new around here and in posting questions in online forums in general so please excuse me if I'm posting in the wrong forum or unknowingly ignore some policy. Any guidance/feedback would be greatly appreciated.

I have been doing BIAB for over a year and a half and I got to the point where I want to take my brewing to the next level. So, I am trying to wrap my head around water chemistry. I live in south east England so our water is super hard, meaning the only thing I have been using to adjust my water profile is some epsom salt when I'm making hoppier beers (apart from lactic acid to adjust the mash ph). Regardless of the fact that this has already greatly improved my beers, they still aren't perfectly to my liking. Specifically, they lose their hop aroma pretty quickly and start to taste overly sweet after a week or two, which I believe is due to oxidation. I bottle all my beers as I do not have the means to keg or to perform a closed transfer so this is no surprise. I read that campden tablets (the form which I have access to is sodium metabisulphite) provide a good way to prevent oxidation as well as neutralising chlorine.
My question is, if I add the sodium metabbisulphite to my water at the beginning of the brewday to neutralise the chlorine, will it stick around until after the boil, fermenting, and bottling is done to prevent oxidation as well or do I have to add more when dry hopping/bottling? According to my water report, I have an average of 0.28ppm chlorine in my water so maybe I don't even need to worry about adding campden in the beginning of the brewday anyway and I can just add some when dry hopping and/or when bottling. I will do some side-by-side test brews in the coming months and will let you know how it goes, but I thought I would get you involved in my brainstorming in the meantime.
[...] I bottle all my beers [...] but I thought I would get you involved in my brainstorming in the meantime.

If the primary source of oxygen in bottle-conditioned beers is headspace (/1/), then reducing the headspace (/1/, /2/) or purging the head space (/3/) may provide the biggest benefit.

After that, other reduced oxygen techniques (sodium metabbisulphite, brewtan-b, ascorbic acid, ...) may provide incremental benefits.

Ingredient selection is also a consideration (/5/), but watch for changes in how those ingredients are made (/6/). Adding fresh yeast for bottle conditioning (/4) may also offer some benefits.

/1/ Dave Miller's Homebrewing Guide (1995), chapter 27.
/2/ "Limiting oxidation: effect of purging headspace O2 in a bottle conditioned IPA"
/3/ "Great Dissolved Oxygen demonstration"
/4/ The New IPA (pp 215-217)
/5/ The New IPA (in many many places)
/6/ "Wait, What is ...?", Craft Beer and Brewing, Winter 2023, pp 38