In general, the amount of time an opponent takes to decide on what play to make is out of your hands. In online play, each player has a clock which runs on each decision they make, and on some sites this seems to run out rather quickly. Pokerstars has a system where a player can ask for more time on occasion, and live play has no set limits on time.

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Having said this, any player at the table can decide to call a clock on an opponent who has already taken a long time deciding. In this instance a floorman will be called over to give a set amount of time to make the decision.

Players usually will be lenient in this type of circumstance, giving 5 or 10 minutes at least before considering calling a clock, as they do not know how difficult a decision there is being made, and all understand the importance of these instances in tournament play. There are phases in online play, as well as live play, where someone might play for time and run the clock down when they are getting close to the money. Even though this may be the case, as a player I always feel obliged to give a person the benefit of the doubt, as it seems impolite to call a clock on a fellow player.

If you do become impatient and want to begin calling clocks on players who you believe are running the clock down, be warned, the moment you have an important decision to make, they are likely to do the same to you.
This is why usually, I will be very accomodating when it comes to players taking time to make a decision, especially if it is for their tournament life.