I don’t know if there’s a thread where the actual values for Acceleration are mentioned. Here’s what I’ve found in testing though.
Every 2 points in Acceleration for a ship displays as 1 point in-game on the Player Info dialog, and corresponds to a rate of change of Speed equal to 1 point per second.
Recall that each point in Speed represents 0.01 pixels per frame, and there are 30 frames per second. Thus every point in Accel (which would display as half a point if half-points would display) gives an acceleration of (1/6000) pixels per frame², or 0.15 pixels per s². For a more useful figure, every 20 points in Accel (which displays as 10 points) gives 0.1 pixels per frame per second, which is 3 pix/s². So 200 points in Accel gives 1 pixel per frame per second, or 30 pix/s².
A ship with Speed 300 and Accel 300 (which displays as 150 in-game) will take 2 seconds, or 60 frames, to reach max speed from a standstill.
Note: any pilot that is not on strict play will have a bonus of 50% more speed than displays in-game. Thus a non-strict play pilot in a ship with 300 Speed and 300 Accel will actually have 450 speed despite what is displayed, so it will take 3 seconds, or 90 frames, to reach max speed from a standstill.
With Speed S and Accel A, the number of frames F that it takes to reach max speed from a standstill is given by F = 60S/A. If you want the time T in seconds, use T = 2S/A.
If you want it to take F frames to reach Speed S from a standstill, a ship needs Accel given by A = 60S/F. If instead you give the time T in seconds then A = 2S/T.
Finally, there is one other thing besides ships in the game that accelerates, namely Freeflight Rockets. I have not been able to nail down exactly how fast they accelerate, but I have a pretty good idea. From my tests, a rocket takes approximately a constant amount of time to transition from its initial speed to its final speed. Perhaps due to flaws in my testing method, it seems to take slightly longer when the rocket has to go through a greater speed change, but not much longer.
When the speed change is around 200, it takes about 3s. When the speed change is around 1000, it takes about 3.5s. When the speed change is around 10000, it takes about 4.5s. I expect that my low-speed-change tests actually underestimate the time, because once the rocket is pretty close to the final speed it can be difficult to tell exactly when it reaches that speed. I timed how long it took until the rocket appeared to reach a constant speed, which through magic of my testing methodology was always exactly the speed of my ship at the time, and I had circular range rings around my ship so I could see exactly how far away the rocket was and thus judged when it stopped getting closer.
If I had to put forth an hypothesis, I’d say the freeflight rocket uses an Accel equal to half its speed, so the total time of acceleration is always 4 seconds (120 frames). But this could be off by quite a bit for a number of reasons. If you have any ideas for more accurate testing methods, please let me know.