She can drive for 12 months from the date she first entered the UK on her spousal visa.
If she entered on March 15th 2017, then she can only legally drive on her US licence until 15h March 2018... which means she has 4 days left.
If she doesn’t have a full U.K. licence yet, then from March 15th she will be treated like a learner driver in the UK and will need a provisional licence in order to drive. She must also adhere to learner driver rules (no motorway driving, L plates always displayed, always accompanied by someone over 21 who has held a full U.K. licence for 3 years etc.).
To note though:
It’s not that her licence expires, it simply becomes illegal for her to drive on it in the U.K. after March 15th. If she does drive after this date without adhering to learner rules and she is stopped by the police she can be charged with driving without a valid licence and driving without insurance.
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