Bankruptcy isn't a get out of debt free card. You can only get debt that you can't realistically repay discharged. The new graduate with a job at Google making $150k isn't getting their student loans discharged.

So the answer to your question is as much as I think they could reasonably repay based on their earning potential after graduation. Which is a reasonable answer to the whole problem except that it hands a huge advantage to rich kids who's parents can write that tuition check.