It depends on your vehicle and your power requirements at that moment.
Each shift is a compromise between power and economy. Rev the engine to a higher rpm when more power is needed, such as when climbing steeper hills or when passing.
Shift at lower rpm for better economy.
In the past most experts would have recommended between 2500rpm – 3000rpm as a good compromise for most 4-cylinder engines, under normal traffic and load conditions.
With our ever increasing attention to environmental impact today manufacturers are setting trends by recommending to consumers to shift at much lower RPM than ever, some as low as 2000 RPM. Larger displacement engines such as V-6’s and V-8’s can be upshifted at even less than that.