yeah, zlk mentioned on #marp the Progear inp at Mog's like a few minutes after you went to sleep, so I went there too and saw the Night Striker recording in the new section. Yay. ;)
The size of the INP changed over the years, but basically a set amount of bytes are read or written for each frame. This varies from version to version.
Prior to 0.35b1 (I think) frames were 16 bytes.
From 0.35b1 to 0.36b15, frames were 32 bytes.
From 0.36b15 to 0.72, frames were 40 bytes.
From 0.72 to 0.84u3, frames were 60 bytes.
From 0.84u4 to 0.87, frame size cannot be determined, due a bug in the input port "rewrite" (should be 120 bytes)
From 0.87u1 on, frame size is 120 bytes.
Add 2 bytes per frame for TGMAME or WolfMAME Plus 0.84 or earlier.
Add 4 bytes per frame for WolfMAME Plus 0.84u4 or later.
Also, remember to subtract 32 bytes for the header, unless the INP is older 0.35b1 (the header was added somewhere around there), or if it was made using MacMAME.
And also note that games that use analogue controls will read/write extra ports as and when the game requests it. This means that for analogue games (including lightgun games) the framesize may vary from frame to frame.
So, using the listed framerate (not all games run at exactly 60Hz):
INP file size is 127,442,080 bytes (minus the header)
divide by 40 = 3,186,052 (number or frame in the recording)
divide by 60 (game's framerate) = 53,100.86666... (seconds in the recording, at 60.00Hz)