It's a small fraction of the velocity, but a significant fraction of the mass. E.g. I think for the reusable missions, the Falcon 9 first stage cuts out around

It's a small fraction of the velocity, but a significant fraction of the mass. E.g. I think for the reusable missions, the Falcon 9 first stage cuts out around Mach 6, but the mass of the first stage (which is mostly liquid oxygen) is about 80% of the mass of the rocket.