From the mIRC helpfile:
/timer[N/name] [-ceomhipr] [time] <repetitions> <interval> <command>
If you specify the -m or -h switch, this indicates that the interval delay is in milliseconds.
Note: The -h switch creates a high-resolution multimedia timer. This type of timer should only be used in critical timer situations since it uses system resources heavily.
A millisecond is 1 thousandth of a second, so there are 500 in half a second.