It doesn't work like a normal loan. So if you make below 20k a year, then you don't pay anything. But you are not "missing" payments or anything like that, it's just that you make less than 20k, so literally nothing is due. If the balance after 40 years(or whatever it is) is still not zero, then the remainder is forgiven by the government. Paying off the student loan has no effect on your credit score as it doesn't even show on credit checks.

My student loan for getting a BSc and MSc in Computer Science at one of the top universities in the UK is about ~12k total. But it's reassuring to know that if I lost my job tomorrow, I wouldn't have any payments to make until I found another job.