Code:
alias mirchelp {
  msg [color:red]#YourChannel[/color] www.mirc.com is a very useful website to visit...
  msg [color:red]#YourChannel[/color] to change your nickname type /nick <new nick>
  msg [color:red]#YourChannel[/color] remember our channels get together on the 4th of January 2006
}


Put the above in remotes (alt +r), File > New > Paste.

After that, start a timer like this:

/timerhelp 0 1800 mirchelp

0 - Means it does not stop, unless you specifically halt it or disconnect/close mirc.
1800 - This is 30 minutes represented in seconds.
The last part tells it to execute the alias I created which is called 'mirchelp'