Hey Hazman.
You already have a command to your disposal, built into mIRC
For example, if you wanted it to post 1 message every 5 minutes, I would use the following:
on $*:TEXT:!announce on:#:{
if (%announce) { return } | set -u15 %announce On | /timerannounce 0 300 .msg # This message will post to your channel every 5 minutes }
on $*:TEXT:!announce off:#:{
set -u15 %announce On | /timerannounce off }
Let me know if this helps