Timing is something you have to feel and understand in the same time. You have to feel it because when you start training you will get to know that one thing is followed by another. For me it's mostly logical to study timing.

Example1: NE versus Human on TR. You scout him and see he is with AM and in most cases he will be creeping with a few militia the green creeps(turtles) or the yellow creep(small trolls and ogres). Now when you scout him(depending on your strategy) you can either harras(dh/warden/other harras hero) or creep(dr/beast/other creep hero). Now the first thing to know is that if he has gone creeping you have to know where he can creep. This is a creep routine. He can't go and creep the red camp(ogres with shockvave and bloodlust) with a few footmans and 4-5 milita right? But he will most proppably go and creep another green or yellow creeps. On TR they are 4 green spots and 4 yelllow(2 trolls+2ogres). When you go and harras him and he is not there, where can he be? Trying to evade your harras. A good scout will help you find him easily. If you have a wisp near one of the green creeps and use it to scout him and then the yellow one and he is not there, where could he be? The other green/yellow creeps or with others words he can use a different route to creep, wich can help him evade your harras. This is only an exaple of early game, where scout is most important. Mid and late game(for me) timing is not random-luck going somewhere, expanding, attacking etc. You have to follow everything in the game - where he was creeping, what items he has(this will reveal, the possible creep location he was creeping). When you get to play the ladder games make sure you follow the game. Everything that hapends depends on how the game will go.

Example2: if you have harrased him well and took advantage you can keep harrasing him in middle game in some way(killing peasants if possible, harassing the AM, etc). Different example is if he has advantage over you - then he can come and attack you and use his advantage or he can put an expo and defend it/attack you afterward. The outcome of the early game will decide the start of the middle game. When you start making some more games, good and focused game you will have to understand and remember : why he did that? Why didn't he do that? Why this, why that...it's mostly logical. So timing is not just random stuff, you have to remember the moves(where he was, where he can go) you have to use your advantage(or defend against his advantage) and scout good. This is the easiest way to learn the art of timing.