LOL. Basically all of it.
You only get tax deduction from the interest you paid. And that is only 2.5k, and only, only if you happen to be making 90K or less [adjusted gross income] [or 185K joint filing]. Which most post residency people won't even qualify for.
So in summary all of your post tax income dollars go to paying on interest & principal of student loans with no tax benefits. This isn't a home loan which the government allows the interest paid on home loans to deduct your Adjusted Gross Income.
Not fair? Taxes aren't fair. And anger shouldn't be directed at lack of a more enhanced credit or deduction for student loans, nay. It should be the original American values that led to the nations founders throwing tea in a harbor. We shouldn't be getting taxed on income in the first place. There shouldn't even be an IRS [*do a review of American IRS history]. We allow government to expand and never should have. But hey, at least we know those dollars are going to such a good cause like DEI, yum, can't have enough of that. Or telling us we can't have gas stoves, or that your water faucets need to flow slower. Or your toilet flushes with so little water you're always staring back at skid streaks.
But to the possible question you didn't ask, if your student student loans got paid off/written off by the gov for completing a 10 year IBR repayment plan... my understanding is the dollar amount they paid... the total X dollars, is considered 'income' and will inflate your gross income, possibly into higher tax brackets, too. Your new gross income will be taxed per routine. So your yearly taxes you expect to pay will be larger than normal, but still, it's better/less than paying the loans off completely yourself... That money is not considered to be a gift for tax purposes.
In summary, talk to your accountant, get a real answer. Thank the government for the privilege of the higher cost of living expense of needing an accountant in the first place.