How long does 35% carbamide peroxide take before it whitens teeth?

It depends: Different people respond differently to the carbamide peroxide. You should notice a difference in a week or two.

Few minutes: Usually 3-5 minutes for 10-14 days.

Depends: 35% carbamide peroxide is very effective on most discolored teeth. You will usually see an improvement after three applications. However, some teeth are very resistant to bleaching particularly teeth that are grey. Since the results vary from patient to patient there is no way to accurately answer your question.