Standard typewriters have 44 keys, assuming you remove the "extra" keys (shift, shiftlock, etc.). Assuming on each slam the monkey only hits one key at a time in a purely random order, the probability of a monkey typing nigger would be (1/44)^6 or 0.0000000138%. This means, every 7,256,313,856 keystrokes, the monkey will statistically type nigger once. Assuming they monkey hits about 300 keys per minute (based on a simple cpm test at a reasonable speed), it will average about 1 nigger per 47.18 years. Given that monkeys can live up to 50 years in captivity, a monkey will likely only be able to type nigger once in its lifespan assuming it starts as soon as possible, is working 24/7 without interruptions, and it lives that long.
So the answer is only once.