Around 9.7 million student loan borrowers became past due on their bills when the payments resumed after the Covid-era payment pause, the Fed estimates.
my guess it takes at least 30 more days to be past due but often 90 and 180 days becomes particularly significant. Alls I can find in the article is:
"By the end of the off-ramp period, the New York Fed estimates that the volume of past-due federal student loans hit 15.6%, with more than $250 billion in delinquent debt."