The loop function is called in an infinite loop from within main.
As soon as loop returns, it is called again.
The length of time between the end of one execution of loop and the start of the next execution of loop calls is a few clock cycles while the return address is popped off the stack, jumped to, pushed back on the stack, and a jump to the loop function location in memory is made.
The length of time that loop takes to execute is a function of what you have it doing.
Assuming you only have the two digital writes in loop the frequency is just under 130 kHz (the period is 7.75us). Around half the time is taken by the digital writes, the remainder by the loop code. If you want to generate high frequencies you would be better served by using one of the timers, for example: Arduino Playground - FrequencyTimer2.
This is one of those cases where measurement does not affect the readings in any meaningful way if you use something like a logic analyzer.