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I searched the Internet and there are three versions of this sentence. Is there any difference between them at any level (grammatical, formality, spoken vs written etc)?
How long is the insurance valid?
For how long is the insurance valid?
How long is the insurance valid for?
I can think of only two issues worth mentioning:
In general, whenever a preposition can either go before its object or be postponed, the former is more formal and tends to appear less in spoken English while the latter is more colloquial and tends to appear more in spoken English. That is true here with sentences 2 and 3, although the difference in this case is pretty minor. (In other words, I probably wouldn't worry about it.)
Some grammarians don't like postponing the preposition at all. This topic has been covered extensively on grammar blogs etc., so I won't go into it here. If you want more info, M-W has an article about the issue.
Otherwise, all three sentences are grammatically correct, can be used in any register, and are common in both written and spoken English.
