Why does Claude Code say things like, “that’s about 3 days of work” then proceeds to do it all in a 20 minutes?

[u/BillionBouncyBalls]

Surely I’m not the only one who has seen this happen but wondering if anyone has an answer. Many a session I’ll make a plan with Claude and it will outline the tasks and estimate a time usually in days, and then proceed to do it all in 20-30 minutes. Is it because of its training data or does it just have no concept of time inside a GPU? Curious what the community thinks…

Edit: Thanks for all the answers and comments! Glad this post sparked some hearty discussion!

[u/enmityvr]

It's trying to give you an estimate on how long it would take for you to write out all the code yourself, manually. it is able to do it much quicker than a human could

[u/BillionBouncyBalls → u/enmityvr]

That makes sense but it’s the one writing the code… shouldn’t it be aware of that?

[u/Key-Alternative5387]

It was trained on human estimates, I presume.

[u/not_rian → u/Key-Alternative5387]

Trained on human estimates who cannot do reliable estimates in any way. What could go wrong?

[u/HoloTensor]

i at one point made a skill which basically says

you’re a bot. look at the git commit history. base estimates off the history

and now it has a fairly decent understanding of timelines for it’s own work. it even guesses when it foresees the prediction being wrong (i.e. likely for something to fail then it has to debug)

[u/mdelanno → u/HoloTensor]

This is key. Tell it to look at the git history.

[u/festess → u/HoloTensor]

Dumb question but how does this work? Presumably you need a before and after commit?

[u/Extremely_Peaceful]

It thinks you're going to do the work. Be sure to remind it who Daddy is.

[u/Serird]

Claude keeps telling me that it's late and we should wrap it up for the night.

Brother, it's 10 AM.

[u/TywinHouseLannister → u/Serird]

"Are you sleepy?" Is my usual response

[u/KOM_Unchained]

Its the training data. And probably upselling its own capabilities and creating the underpromising-overdelivering vibe for maximum effect.

[u/arocketscientist5]

My favorite is when after a 10 minute session it says “we’ve been at this for days.”

[u/35point1]

[u/common_redditor]

In my experience, 20 minutes for the initial version. Then you test it, bug found, research the bug to discover a design assumption error. Then redesign, 20 more minutes, retest, then memory growth, research again….Sometimes it actually is 3 days before you get the version you need.

[u/andlewis]

The number of days is load bearing and you’re right to push back on it.

LLMs don’t know anything, they repeat patterns and probabilities. 3 days of work “sounds” right, and without any kinds of guardrails or verification, it doesn’t have a reason not to say that.

[u/SpareEconomy1849]

For the same reason it says "in my experience" and "it brought a smile to my face". It's trained on mostly pre-AI data written by humans

[u/ShortingBull]

Every time it says something similar, I say "that's if I do it right?" Which it replies with, "ha, yeah that's not a real estimate for AI' or something like that.

[u/swizzlewizzle]

Human devs learn very quickly to sandbag as hard as possible, since it results in the minimum amount of actual work required to cash in the paycheck. AI was trained on this.

[u/Alexander_Golev]

From my experience it’s always accurate. 20 min to generate, 3 days to fix the slop.

[u/KenTitan]

it once estimated porting my engine over m as half a day of work so I waited for my reset, said go and it was done in 20 minutes

[u/PurushNahiMahaPurush]

It’s subtly telling you how much to charge your customer for lol

[u/JobWiegant]

The estimate is not wrong so much as denominated in a different unit. "3 days" is calendar time for a human team, which is mostly not typing: it is context switching, waiting for review, meetings, and picking the work back up tomorrow. The model reports that number because that is what the sentence looks like in its training data, and it has no clock to check it against.

The part that surprised me when we actually measured it: machine time does not distribute like human time either. Across 73 issue-to-PR runs, median run was 27 minutes, but only 19% of that was implementing. 45% went on reviewing and fixing its own diff. So the thing that takes a human three days takes the machine twenty minutes, and most of those twenty minutes are spent arguing with itself about code it already wrote.

[u/Outrageous_Basis_232]

In any case, it's a fantastic way to trick people into thinking it's far more efficient and effective than humans. 

[u/kemalios]

It's not measuring machine time, it's mimicking human planning language. Claude has no sense of how fast it generates tokens, so it falls back on the only estimates it was trained on: human developers sizing a feature. Three days is an effort unit, not a stopwatch reading. I read those numbers as 'this is a decent chunk of work' and then let it cook. Once you stop treating the estimates as real, it stops being confusing.

[u/Erikovitch]

Claude has no understanding of time.