Really? What difference does it make? Make it 1 minute. Make it 100 minutes. Make it 100,000,000 minutes. When the spell is a personal cooldown whose cooldown duration matches or is greater than the debuff duration, the spell will always be limited by the personal cooldown, making the debuff redundant and unnecessary. Do you need me to flowchart the logic out for you?
Here's your choices if you want it up every attempt:
Make it a exhaustion-style debuff that clears on death. If that's the case the developer will have to contend with people suiciding, battle resing, and summoning multiple Doomguards for as often as the have battle reses available. In addition they'll have to deal with Doomguards being summoned after every death in unrated BGs.
Second choice? Scrap the debuff idea and just lower the cooldown, but in turn, you'll probably have a significantly less powerful doomguard cooldown to balance the ability to bring it out more than once an encounter.
In the end it comes down to: Do you want a weak Doomguard cooldown that's up every single attempt, guaranteed, or do you want a monsterfully powerful Doomguard cooldown that probably won't be available every attempt?