[black6]

It depends on the number of PIDs and the speed of the data acquisition system. When I worked on the RS-25 engine project the engine had hundreds of PIDs, the low speed DAS was 250 samples per second, and the high speed DAS was 250,000 sps. The high speed DAS generated so much data that it was only started a few seconds before engine start so the recorders wouldn't fill up before the end of the tests (which were typically 500 seconds).

EDIT: something to add is that not ever PID was tied into the high speed DAS–only a couple-few dozen important PIDs.