You use what you had, but change "reset=40" to "reset=601". You can't make a spell only cast at a certain time, you can make a spell not cast until its cooldown is up. You just have to press the key every 40 secs. You should see the buff time out either in your buffs section of your UI or with scrolling text in the middle of your screen. Again, just put the reset time as the cooldown time (I add 1 sec to make sure).

Hope that helps.