I sat down an wondered what is the best way to do this oneday.
I think this is the best idea... It will be set not exactly when that time comes... but after the next ping.

Best used with mIRC's "Cancel away on activity" option (found in Mirc Options-->IRC-->Options

Code:
on 1:PING:{ if (!$away) && ($config.autoaway <= $idle) { away Auto-away after $duration($ifmatch) } }

alias -l config.autoway return 1800


The config.autoaway is the amount of time in seconds... its currently on 1800 which is half an hour

[02:16] * Titanic has quit IRC (Excess Flood)