"Timing ends when all sprites are invisible after touching the axe"
Is that frame counted to the timing?
e.g. If the axe-touch frame is 1000, is the length of the run 999 or 1000 frames?

It's the first frame you touch the axe, so shouldn't it be 1000 frames?
Sorry, I'm not really a Mario Bros. speedrunner.

Yes, the frame is counted. If you touch the axe a frame 1000, the run is 1000 frames long

Hello there o/

To answer your question, you should go frame by frame when Mario is touching the axe. There is literally 1 frame where ALL THE SPRITES disappear (Mario, Bowser and the axe itself)

And that's gonna be the final frame of your run.

Hope this helps.

Happy new year!