Is it possible to obtain a Big O estimate of Math.random()?
I'm not sure this question makes much sense. There's no variable size input to increase gauge complexity against - you make a call (with no arguments) and you get an output.
Are you asking if the Math.random() method takes longer for successive calls? Or if it's just slower than it should be?
Remember that even algorithms with O(1) complexity can take a long time - it's just that the length of time they take doesn't depend on how much data is involved.