Under normal driving conditions, an alignment once or twice per year is recommended. However, this recommendation is very fluid. If you drive on particularly rough roads, or have a habit of contacting curbs (those pesky things come out of nowhere), then you should probably be on a first-name basis with your alignment professional.
Keep in mind that there’s no guaranteed length of time that a vehicle will stay in alignment. If you hit a New York City-sized pothole leaving the shop, the alignment could be undone right then.
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