Travis has the right idea on the sys_trigger job list. Every "timer" workflow activity creates a record in the sys_trigger table with the name 'WFTimer' + the sys_id of the instantiated workflow activity.
You could create a business rule on your sc_req_item table with the conditions for that workflow that specify "if this value changes and it's for this particular request, made a modification to the sys_trigger table."
Your flow would look like this:
User changes disable time
Business Rule fires
Business Rule looks up your current context record
Business Rule finds the timer actively running in the current context record
Business Rule finds the sys_trigger named 'WFTimer' + the sys_id of the timer activity actively running in the current context record
Business Rule changes the next action time to be whatever is in the Request Item.
What might be easier and more scalable is a loop in your workflow, provided you never decrease your disable time. It would be something like this:
Run Script activity sets a workflow scratchpad variable equal to your wait time duration
The timer activity can accept a script, so you would set the answer to be equal to your scratchpad variable
The timer runs out and then passes through an if script: "Is the time in the request item less than the current time?"
If it's less, that means we're good to disable. If it's still greater than the current time, the if script sets the scratchpad variable to the new duration and loops back onto the wait condition.
