It's 10% of discretionary income, which is the amount that excess 150% of the federal poverty level. Nonetheless, the law also allows for hardship forbearances that allow people to stop paying without defaulting. Personally, I think the law is a backdoor subsidy done in the worst way. It increases stress on families who can't afford the tuition for 20 years to give a subsidy to the university. It incentivizes high tuition and keeping students on the treadmill, with no concern for quality. I don't particularly mind if tax money helps pay for an esoteric major, but I expect it to produce a capable scholar in that field.