Given that it takes myself quite some time to resolve my combat turns, I never felt it would be a good idea to introduce timers in my games. If a player consistently takes very long to decide on what to do, I've sometimes ruled that she's taking the 'delay action' for the time being. Looking up rules is something I try to avoid during combat. I feel it's better to make a ruling on the fly to keep the action going and look it up after combat or even better after the session. As always, there are exceptions.