About 30% ever graduate. The two curves — odds already spent and odds remaining — cross at roughly month 27, the moment a company has a better chance of being already funded than of ever being funded. By month 36 the hazard is below half its peak and 72% of eventual graduates are through.
A bridge round is a lifeline, not a tombstone: among companies still B-less at month 24, those who took a smaller interim round went on to raise a real B 25.8% of the time versus 15.8% for those who didn't. Round size matters too — the $8–18M A band graduates best, an inverted U.
Every claim above comes from work LeanScale published. These are the sources.