One way to do this would be to create a new conditional task that gets reevaluated by a Self conditional abort. Within the Update method you'd return false as soon as the timer expired. Take a look at this page for how to create a new conditional task:
This topic is divided into two parts. The first part describes writing a new conditional task, and the second part (available here) describes writing a . . .
IsTimerUp being reevaluated? With the Self conditional abort it should be reevaluated every frame. As soon as IsTimerUp returns false it will abort the right branch.
What does your tree look like? What does your timer conditional task look like? The method that I described above is the best way to do it for your particular situation. I can add a timer conditional task to the next update.