What GOV.UK says
The official page sets out the format — 45 minutes for 24 questions, £50 — and, on taking the test, states that you do not need to take it again if you have passed it before.
That is the closest thing to an answer the government publishes: a pass is treated as something you already hold, not something that lapses on a date.
What it does not say
There is no published expiry date, no stated validity period, and no detail about how long evidence of a pass must be kept. We are not going to invent any of those. Where the government does not state a rule, the honest answer is that it does not state one.
What to do in practice
Keep the evidence of your pass with your immigration papers, in the same place as your passport and your visa documents. If you passed some years ago and are applying now, check the official page first, and take advice from a registered immigration adviser if anything about your case is unusual.
If you have not passed yet, the practical answer is simply to sit it once and keep the result safe.
If you still have to pass it
The test is 24 multiple-choice questions with 18 correct to pass. Every question in our bank is free to read, and the first practice test is free to sit.