After wearing heavy armour for more than 1 hour per two hit dice (minimum 1) the character becomes fatigued. Proficiency in heavy armour doubles the time. If the creature is a quadruped this time is quadrupled. If a fatigued character continues for the same period of time he becomes exhausted. This time can be extended using the Endurance feat.
Any shield armour bonus is added to your touch AC as well as checks to resist bull rush, disarm, grapple, overrun or trip. Any circumstance which causes you to lose your dex bonus to AC results in the loss of these shield bonuses.
If you fight defensively (or use at least 2 points of combat expertise) your shield bonus is multiplied by 1.5 for the heavy shield or doubled for the tower shield.
Drawing a shield is the equivalent of drawing a weapon. It gives no bonus to AC unti it is readied, which is a move-equivalent action that does NOT provoke an attack of opportunity and can be done whilst moving. Thus, drawing a shield and readying it requires 2 move actions.
As with heavy armour, carrying a heavy load for more than 1 hour / 2 HD makes the individual fatigued. If the creature is a quadruped this time is quadrupled. If a fatigued character continues for the same period of time he becomes exhausted.