So, to simplify;
ECM is a module that you mount on your ship. It works continuously (until destroyed) by reducing the range of the enemy fire control systems. The more advanced the ECM, the more the enemy systems are affected. You generally dont have to mount more than one ECM, unless you want redundancy, but there are usually better things to do with the weight and space.
ECM works best against beam fire controls. Usually beam FC targeting range is closer to the actual range of the weapon, so if you have a laser with a 250,000 km range, and the fire control is degraded to 125,000 km, the enemy wont be able to fire until 125,000 km.
Missile fire control are smaller and lighter, so it is MUCH easier to build a missile FC that seriously outranges the actual endurance of the missile. Most veteran players do this routinely for a couple of reasons. First, its means you can use those FC for longer periods of time, since your missiles will take a while to catch up. Secondly, there are other units using ECM, so its a cheap way to negate NPR ECM instead of mounting expensive ECCM systems.
So, if your planning on using it, one ECM on the ship and your good. If the bad guys are using it, build your missile FC to outrange your actual missile range by a good amount. For example, if your missile range is 50 million km, build your FC to target out to 75 million km. Its cheaper and easier than using ECCM.