It depends on how you play. If you are just focused on completing the main story as quickly as you can you will complete it much faster than if you complete all the side missions and do all the other things in the game. Here is a general break down:
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Main story: ~20 hours
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Street Crimes: ~5 hours (most of this time is in getting to the crime; once you're there it usually takes less than 10 minutes per street crime)
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Collectibles (Film Reels, Cars, etc.): A very long time
For help finishing L.A. Noire, use these case guides.
