Use /timer 0 900 msg #channel Message
. That would message #channel the word "Message" every 900 seconds (15 minutes). The timer will turn off when you disconnect, though, and will also continue when you are not on the channel.
To have the timer activated automatically when you join, and turned off when you leave use this:
on *:JOIN:[color:green]#channel[/color]:if ( $nick == $me ) timerm $+ # 0 900 msg # [color:green]Message[/color]
on *:PART:[color:green]#channel[/color]:if ( $nick == $me ) timerm $+ # off