It took me 10 years, though I wasn't attending classes the entire time, to be sure, and part of that was at a community college, then a university, then back to a community college, then back to university. A lot of that time, I was not accruing debt, but I badly needed the loans later on when aggressively pushing to finish. I then completed my Master's in the idealised 2 years. Sometimes life is very hard, and it's very hard to take a full course load or avoid interruptions in your studies. So your policy would blindly result in the world having one less successfully employed person making twice the median income of my township and having to write checks to the IRS at tax-time instead of getting money back. While the student loan system is deeply broken and badly needs reform (it needs to be tied to the field of study, to employment prospects, and to the school--I played it smart and stayed at community colleges and State Ag schools, but debt was still needed), the reality is some simple policy like that may well hurt the chances of as many people to become productive adults as it punishes the chances of useless layabouts.