How long should I try an acne product before changing it?

Acne treatment takes weeks, not days. Many treatments need around six to eight weeks of consistent, directed use before you can fairly judge early progress. I also want to see whether the number, type, or severity of breakouts is changing rather than judging success only by whether the skin is perfectly clear.

Acne treatment needs enough time to judge

A few days is usually too soon to decide that a treatment has failed.

Irritation changes the plan

Irritation, deep pain, worsening inflammation, or scarring changes the decision. I would not keep increasing strength just to finish a fixed trial period.