Just use combat rounds
This battle is against time. Have characters roll initiative and proceed in order, describing their actions according to the chart Actions in Combat. It's perfectly acceptable to explain to the players beforehand that this is not a combat encounter and that you're using combat mechanics to simulate a ticking clock, especially if they're unused to rolling initiative at any other time.
Also, be sure to roll initiative for the candle. (I suggest the candle have an initiative modifier of −5.)
After the trap's sprung or disarmed, declare the encounter over, and give them time to catch their breaths using out-of-combat time.
On the RAWness of this practice
Pathfinder measures noncombat time in rounds for durations of spells and special abilities and how long it takes to use some skills and a variety of other things. That's completely within the rules. Don't feel as though you're doing it wrong if you've to reference rounds and turns during a noncombat stretch.
Further, Initiative says that
At the start of a battle, each combatant makes an initiative check.
...yet the DM determines what a battle actually is, and that can include a duel of wits or a race against time or whatever you come up with.