Your skin takes a month to answer. That's why you never know whether the cream worked.

I opened a makeup manual looking for technique and found a number that explains an entire frustration.

A new skin cell takes about a month to reach the surface.

A month. Thirty days between being made down there and showing up in the mirror.

So what you're looking at this morning isn't the result of what you did yesterday. It's the result of what was going on a month ago.

The journey one cell makes

Skin has three layers. The deepest, the hypodermis, holds fat, blood vessels and nerves. In the middle is the dermis, made mostly of collagen and elastin — that's the layer that decides how skin responds and how it changes with age. On top is the epidermis, which is the part that gives immediate visual clues: it's what you're looking at when you look at your face.

The epidermis has several layers of its own. New cells, called basal cells, are made in the lowest one and travel upward. They pass through a hardened layer, reach the surface and flake off — and on the way they lose moisture and oxygen. By the time they arrive, they're dry.

Which is why moisturiser works in two different ways at once, and this is the part I found clarifying. First, it fills the gaps between those already-dry cells, and that's the effect you feel immediately — smooth skin. Second, it forms a barrier that holds water in. The first effect is instant and cosmetic. The second is slow, and it's the one that changes skin.

Two things happening out of the same jar, on two different clocks.

What that changes in practice

If a cell takes a month to climb, then:

You can't judge a product in a week. In seven days, no new cell has finished the trip. What you're assessing is the surface effect — whether the cream is comfortable, whether makeup sits better on top of it, whether it irritated you. That's useful information, but it isn't the question you were trying to answer.

A month is the minimum for having an opinion. I'd say six weeks, to leave margin.

Changing two things at once throws the month away. If you started a new cleanser and a new serum on the same Monday, and one of them stung, you have no way of knowing which. One at a time is tedious, and it's the only version that gives you an answer.

And there's a consequence that's almost a relief: if your skin is odd today, it isn't because you got today wrong. It could be last month, the weather, hormones, travel. The book lists exactly that — hormonal fluctuation, stress, medication, travel and the change of season — as things that make skin behave differently from one day to the next without you having changed anything.

The thing I didn't know about pores

This one is more honest than I expected from a book with a brand name on the cover.

No cosmetic changes the size of a pore. None. The book says so plainly.

And then it explains how toners and astringents make a pore look smaller, and the mechanism is slightly disconcerting: they irritate the skin very slightly, the skin swells a little in response, and the pore becomes less visible.

So: the effect is real, it's temporary, and it's a reaction to a deliberate irritation.

That isn't a reason to throw the toner away. It's a reason to know what you're buying, and above all how much alcohol is in it. The book gives a scale I'd never seen written down:

• skin freshener — under 10% alcohol
• skin tonic — up to 20%
• astringent — up to 60%, plus antiseptic

Sixty per cent. That's a product for very oily skin, or for one spot, and not for the whole face every day.

And the best toner on the list is free: cold water. The book says it without hedging — a splash of cold water closes the pores and improves texture, and it suits any skin type.

Three places where I don't follow the book

Here I have to be straight with you, because the book is from 2008, it's American, and some of it hasn't aged well.

One: the book says the body is 80 per cent water. It isn't. I've already published the right figure here, and it came from a food science book: an adult body runs around 60 per cent water, and it varies a good deal from person to person. Eighty is closer to a newborn. It doesn't change the advice to drink water, but it's a wrong number and I'm not going to repeat it just because it's in print.

Two: "eight glasses of water a day." That number has been circulating for decades without firm ground under it. What's actually solid is simpler: a large share of your water comes from food, and thirst is a reasonable guide for most people. Drink water. Just don't treat eight as a clinical target, because it isn't one.

Three, and this is the one that matters here: the book recommends SPF 15 in winter and 30 in summer. That was written for a four-season country. Here the UV index doesn't drop that way in winter — July in Belo Horizonte isn't January, but it isn't February in Boston either. I use 30 or more all year, and this is the sort of decision worth having with a dermatologist rather than with me. I'm not one.

The part that's just kindness

Two small things from the book I adopted immediately, because they cost nothing:

Rinse with warm water, not hot. Including around the eyes.

Pat dry. Don't rub. And the reason is more concrete than I'd assumed: rubbing, or dragging a towel across the face, creates tiny abrasions on the surface, and those abrasions become irritation, redness and even swelling. The redness you've been filing under "sensitive skin" may be the towel.

It's the kind of thing nobody tells you because there's nothing to sell.

What I'm going to do

One new product at a time. Six weeks. And a photograph in the same spot of the house, in the same light, on the first day and the last — because memory is terrible at slow comparison, and the mirror lies a little every day.

If you read why a brush is chosen by texture and not by price, it's the same logic: the tool and the time do more than the label. And if your subject is eyebrows, there's a quirk of perspective that explains why almost everyone over-plucks one side.

If you'd like the rest of what we're putting together for the season, I've gathered it all on one page, with a table checklist to download.

And if your December runs more through the table than the mirror, The Art of Napkin Folding has the classic folds step by step, with the starch detail that decides whether a fold stands up. R$47.00, lifetime access.

Tell me: can you wait a month before judging a cream, or do you give up in week two? I used to give up on the Wednesday.