I keep seeing images like this:

All I want to know is the length of the 'duty cycle' (or the Hz of analogWrite(127); ) on an Uno. Not very important at all, actually.
EDIT: it's 1.0204081632... milliseconds. I have to wait a few minutes before I can edit the title and officially solve it.
EDIT 2: it's twice that on pins 9, 10, and 11, (and with the exception of the Leonardo, pin 3).

look at the reference

You could use pulseIn() to measure it. Or, since all PWM pins toggle at a known frequency, you could simply find out what the frequency is for the specific pin on the specific Arduino and calculate the duty cycle.