MIDI beat clock time in ms?

[superfury]

Afaik 24 of them are used per quarter note. How do I know how long a quarter note lasts (in ms/us)? I can't find anything in the GM specification (using it for my MIDI card emulation)? 0xFF is used in .MID files, but invalid (causes a reset) on normal hardware? What command is used to change/set the speed? Is this permanently set to a period of time or is it to be set by software via MIDI I/O (port 0x330/0x331 or 0x300/0x301)? (The BPM or ms/us taken by a MIDI beat/clock)