how fast does loop() run in Arduino
Loop runs as long as loop needs to run. 

Instructions in a CPU run sequentially. The more instructions there are the longer it takes to run.

The more code you put in loop the longer loop will run. 

There are two ways to know how long each iteration of loop will take:


Profiling: Actively time each iteration of loop, though be warned, the act of timing will affect the amount of time taken.
Cycle counting. Compile the source into assembly and total up the number of clock cycles needed for all the instructions (including calls to functions). Arduous for small loops, a gargantuan task for anything else.


Also note that many external factors can affect how long loop() takes to run - such as serial communication speed, etc.