In Kittens Game, when does refining catnip to wood get a huge boost in late-game

In the early game

Woodcutters win. They're giving you 0.015 wood per game tick, whereas farmers are giving you 1 catnip per tick. In a hundred ticks, the farmer has earned you one wood, whereas the woodcutter has earned you 1.5.

As you upgrade

The Mineral axes upgrade is +70% wood. Mineral Hoes is only +50% catnip. Iron Axes/Hoes are 50%/30%, and from Steel onward, Hoes aren't getting any more upgrades.

When you get the Catnip Enrichment upgrade

You've probably already got the Mineral/Iron upgrades mentioned above. A hundred ticks of 1.8 catnip is 180 catnip, or the equivalent of 3.6 wood from a farmer. Those same hundred ticks of a woodcutter with Mineral and Iron are worth 2.2 wood. Even Steel only pushes it to 2.7.

However, by this point, you've probably also got Lumber Mills and Aqueducts. Lumber Mills are providing a +10% bonus to wood, Aqueducts a mere +3% to catnip. With Steel Axes, Lumber Mills can quickly put Woodcutters back on top.

Storage

Another big reason Woodcutters will tend to win, is storage. This game isn't going to be over in an afternoon; at some point, you're going to start leaving it for a while to build up resources. Your catnip storage is never going to be fifty times your wood storage, so if you plan to leave the game for an hour, it's better to have your kittens working on wood directly; farmers will hit your catnip cap quickly, and need you to manually hit the refine button.

Workshop Automation

Related to the point about storage, there is an upgrade in the game that allows automating some of the workshop crafting tasks while idle. Every so often, it will craft some of your Wood into Beams, Minerals to Slabs, and with another upgrade, Iron into Plates, as long as your storage of those resources is full. This is great for leaving the game overnight, but one thing it notably doesn't do is refine catnip. By having your kittens working on wood directly, you'll be more likely to have your wood supply back to full every time this effect runs. Additionally, by now, you've potentially gotten the fourth Axes upgrade, as well as the first couple of Saw upgrades that improve your Lumber Mills

Skills

So, even with all this, wouldn't it still be better to at least focus on Catnip during the period right after you get Catnip Enrichment, at least while you're actively paying attention and refining? Sadly, no. Because your kittens have skills, and thus get better at a job that they keep working at. Shunting your kittens back and forth means splitting their skill growth, resulting in a population of kittens which take longer getting to Proficient or Master at either job. The bonuses for the last two skill levels are quite impressive (50% for proficient, 75% for Master), so keeping your kittens on the same job is generally a good idea.

TL;DR

Kittens should be made farmers only as needed to keep your population alive. Refining spare catnip won't hurt (unless it'd be better spent on a Catnip Field or Pasture), but the only time you should be focusing heavily on refining catnip, is if you're playing in Iron Will mode.

The list of achievements and how to get them can be reviewed on the Kittens Game wiki.

Unicorn Conspiracy

Find one unicorn (from hunting or random event in Iron Will)

Uniception

Find one unicorn tear (build a ziggurat, research theology, obtain Faith, and sacrifice unicorns)

Sins of a Solar Empire

Find one alicorn (requires Sky Palace)

Anachronox

Find one time crystal (sacrifice alicorns or trade with Leviathans)

Dead Space

Find one necrocorn (requires Marker)

Iron Will

Build a mine without ever buying housing in current game

Uberkatzchen

Build a warehouse without ever buying housing in current game

One Hundred Years of Solitude

Build a steamworks without ever buying housing in current game

Virgin Soil Upturned

Build 45 pastures without ever buying housing in current game

Atlas Unmeowed

Build a magneto without ever buying housing in current game

Meow Meow Revolution

Build a factory without ever buying housing in current game

Space Oddity

Research Moon Mission without ever buying housing in current game (Regular Version) Research Moon Mission without ever buying housing in current game with less than 10 paragon (Insane Version, has Star)

Jupiter Ascending

Complete the Orbital Launch mission before the end of year 1

Shadow Of The Colossus

With exactly one kitten in the current game, build a Ziggurat

Sun God

Accumulate 696,342 faith

Heart Of Darkness

Have more than one zebra

Winter Is Coming

Kill 10 kittens in current game

You Monster

Kill 100 kittens in current game

Super Unethical Climax

Click on the "Gather Catnip" button 2,500 times with no more than 2.5 seconds passing between each click having had at least one "extra" kitten (kittens > 0) at some point in the current game

Lotus Eater Machine

Obtain at least 1 karma by resetting game

Serenity

Have 50 kittens with none dying in current game

Utopia Project

Have more than 35 kittens and a happiness score of 150% or greater. (Regular Version) Have more than 35 kittens and a happiness score of 500% or greater. (Insane Version, has Star)

Cathammer 40K

Have 40,000 total years played (It's in Stats!). (Regular Version) Get to game year 40,000. (Insane Version, has Star) (Please note that years gained by skipping ahead by shattering time crystals DOES NOT COUNT!)

Limitless Clicker

Have clicked 100,000 times (It's in Stats!) Have clicked 1,000,000 times (Insane Version, has Star)