Calculating power requirements of RGB LED strips
There are way too many types of LED strips (e.g. individually addressable or not, I2C, SPI or PWM interface, integrated current limiting resistors, ...) 
A wide variety of LED chips are used in such strips (5 mA, 20 mA, 500 mA or even 1 Ampere per color, and SMD 3528 through to high-power LED per color), for any general rule of thumb to be applicable.
Also, the number of LEDs in a strip is yet another variable. 


In short, there is no such thing as a "typical" RGB LED strip. 

An LED strip manufacturer will typically list rated power for the strip, that is the best recourse for determining power requirements.