I had tried 2/3/4/5/6/7/8 seconds without success. Putting it to ground the entire time kept it in bootmode. Removing it drops it from boot. I can video it if you want. 3 ECUs all with the same story.
I remember clear as day because the first time I bricked it I was panicking, had tried the method as documented (3 seconds during boot) and it never worked. The first time I put the resistor to it and kept it there, it worked.
You can't R/W the flash if you keep Pin 24 grounded on a 29F800. For the simple reason that Pin 24 is A0.
Also, once the MCU is in boot, that's it. Until the uploaded program reboots it, it will stay in boot. If the behavior you see is any different then you have a problem in your toolchain or hardware.