Literally, when? Under what circumstances does an engine need to be timed? And how do you know if you can use the static timing method?
Engines need to be timed (or re-timed) whenever the timing is off. Makes sense, right? This can happen naturally and gradually as the car is driven, and will often make itself known by causing the pinging or knocking sounds that are the result of the engine's cycles running out of sync. When the timing is slightly off, the fuel and air won't be precisely injected to mix at exactly the right time and place. At best, the car will run a little less efficiently than usual, and the engine will probably be somewhat noisy. More likely, the car will run noticeably poor, fuel economy will take a dive and sounds coming from under the hood will express the engine's displeasure. And at worst, the engine will slip so far out of time that its moving parts can actually crash into each other, causing the need for a full engine replacement or at least a costly and extensive valve-repair job.
Because the engine's precision can start to wane over the course of normal operation, chances are you'll have to deal with an engine timing issue at some point during a car's lifespan. If the timing is off, it can cause a lot of drivability issues. If the engine knocks, the timing might be too advanced (but since other problems can cause this symptom, it's best to do a bit of troubleshooting before experimenting with repairs). And often, when the engine is taken apart and repaired, it'll have to be re-timed as part of its reassembly. A good example of this is a timing belt replacement. Since timing belts have a lifespan of about 60,000 miles (96,561 kilometers) (give or take), chances are, if you own your car for a few years, this procedure will eventually be part of its required maintenance. The timing belt itself is just a few bucks, but if the rubber wears through or a tooth wears down, it'll probably crash at least a couple valves in the engine. A standard four-stroke engine (like the one you're probably dealing with) is designed so the crankshaft rotates twice for each time the engine fires. If you're looking at all the engine's moving parts and scratching your head, you might not know if that sounds like a lot, or like not very much at all. One thing's for sure, though -- it's a precise machine and there's no room for error.
That said, because static timing is considered to be pretty easy for someone with even basic mechanical skills, it's standard for the level of engine-related chores that can be completed in an average home garage. Static engine timing, because of its relative gentleness, is also recommended prior to starting a new or rebuilt engine for the very first time. If you bought a crate engine to install in your weekend ride or you had a machine shop make some expensive tweaks to the engine on your Harley, static timing is probably the best way to prep for your maiden voyage.