It’s been a long while, but in the C-5 where we did lots check speeds on standing take-offs, 100 knots was about 35 seconds into the roll, plus or minus for extremes. We had a standard of not more than 3 knots below 100 at the end of the scheduled time. Manual had a graph to figure the more accurate number. A friend used to say 100 knots at 3000’, on task-off or landing, you should be at 100 knots after 3000’ or no more than 100 knots with 3,000’ remaining. Funny, the one take-off I remember well, it was apparent to me in the jump seat that it was going to be close. Sure enough, 97 knots at the time and it might 96.5. It’s amazing how much distance goes by accelerating above 120-130 knots.