How long should a 150 and 250 word answer actually take?

The paper is three hours. The question of how to spend it is usually settled in the exam hall, badly, by someone who has never timed themselves.

The arithmetic nobody does until it is late

A GS paper is 20 questions, 250 marks, 180 minutes. Ten questions at 10 marks and ten at 15 is the usual shape — roughly 150 words and 250 words respectively.

Spread evenly, that is 9 minutes a question. But the marks are not spread evenly, so neither should the minutes be: around 7 minutes for a 10-marker and around 11 for a 15-marker lands you at 180 with nothing spare.

Nothing spare. That is the part worth sitting with. There is no reading time, no thinking-about-the-first-question time, and no recovering-from-a-bad-start time in that budget. Every minute you spend deciding what to write is a minute taken from a later answer you have not read yet.

Why most practice does not reveal this

Writing one answer well in twenty minutes teaches you that you can write a good answer. It does not teach you whether you can write twenty of them. Those are different skills, and only the second one is examined.

Three things only show up when you time yourself honestly:

Thinking time is not free. For most people it is two to four minutes on a question they half-know, and nearly zero on one they know cold. The variance is the problem, not the average.
Pace decays. Answer 18 is slower than answer 3, and nobody plans for that.
The structure you default to under time pressure is rarely the structure you use when practising one answer in a quiet hour.

Measure thinking and writing separately

This is the single change that makes timed practice useful rather than merely stressful. If you only know a question took 11 minutes, you know nothing actionable. If you know it was 5 minutes of thinking and 6 of writing, you know exactly what to fix — and it is almost never the handwriting.

A rough reading of what the split tells you:

Thinking high, writing low — you do not know the topic well enough yet. More revision, not more writing practice.
Thinking low, writing high — you know it and cannot compress it. Practise structure: the intro that is one sentence, the body in parts, the conclusion that does not restate.
Both high — you are at an early stage with this topic and the honest answer is that it is not ready for timed practice.

Gravitropic’s answer-writing timer splits the two phases for exactly this reason, and keeps the history so you can see the numbers move over weeks rather than guessing from how a session felt.

A practice progression that works

Weeks 1–2: untimed, structured. Build the habit of a shape — intro, body in parts, conclusion — without a clock. Learn what good looks like first.
Weeks 3–4: single answers, timed. One question at a time at 7 or 11 minutes. Record thinking and writing separately. Expect to overshoot badly; that is data.
Weeks 5+: sets, with laps. Four or five questions back to back, lapping each one. This is where pace decay appears, and it is the only way to find it.
Monthly: a full section under real conditions, same time of day as the paper.

What the clock will not do

It will not make a thin answer good. Timing is a constraint you learn to write inside, not a substitute for knowing the material — an aspirant who hits 11 minutes on every answer and has nothing to say in them has optimised the wrong variable.

Use the clock to find out what you can do under the real constraint, then go and fix whichever half the numbers point at.