Zerg vs. Protoss Timings
In general, ZvP has one of the most structured gameflows and is heavily scouting dependent. In current pro play, there are very few things that can be done differently without being punished. Optimizations are small with the forge first fast expand build, and everything you can do to get yourself ahead ends up adding up by the time your chosen tech starts to be aggressive. In general, Zerg should always be one gas ahead of the Protoss player, and ideally should have enough drones to make the gas/min balance effective.
Early game in this case is before Protoss gets their first Zealot out, and before you start down your Lair tech path (assuming that you're not cheesing).
There are several important things to scout, and the importance depends upon the opening build that you chose. The most important thing at this point is to see if your Protoss opponent went with a Forge first build, or a Gateway first build.
Against Forge First
If you scout a Forge first build, look to be able to expand to your third gas to be able to keep up in economy, and know that unless you manage to do a large amount of damage with a Zergling runby or a fast Hydra push, you're going to need to continue in the game with a mindset of winning in mid/endgame.
Against Gate First
Against any sort of 1 gate or 2 gate build, you need to be careful. As a general rule of thumb, you should think about the Protoss player going either 2 gate zeal (which you should be able to see with your scouting Overlord) or 1 gate tech. You should get your Hatchery at your natural as soon as you can, and try to get your economy up to speed. Make sure that you get either Hydralisks or Mutalisks whenever the Protoss goes 1 gate tech to counter the first Corsair.
Against 1 Gate Tech
At this point, you're looking to either gain map control, or defend against any sort of one base play Protoss can throw at you. If he went for a fast expand build, you can fake a threat by placing and canceling a fast Hydralisk Den or a Lair if the scouting Probe is still alive, thus forcing him to build more Photon Cannons.
Midgame usually hits whenever Protoss gets his first Corsair out. This first Corsair is key for Protoss, as it allows the Protoss player to see what kind of tech you have, what kind of economy you have, and adapt from there. For the Zerg player, midgame usually hits when you have Lair tech, as it comes out at about the same timing that the first Corsair comes out.
Scouting at this point of the game is very difficult for the Zerg player, which is another advantage of the Zergling runby. Assuming that Protoss has at least one Corsair and one Dragoon, you might consider either attempting a runby, or suiciding an overlord to see what kind of tech path Protoss is going to attempt.
Besides attempting a runby or suiciding an overlord, Mutalisks or Scourges can be effective in trying to spot the Protoss tech path.
Many of the adaptations at this point in the game depend on how well you think you are doing relative to your opponent, as well as what you have scouted. At the beginning of the game, you should have been working on your economy by building as few Zerglings as possible. At this point, you should be considering when you should start to power units. The longer that you can get away with just teching, upgrading, and building Drones, the stronger your economy will be when you decide to attack.
Deciding when to attack and when to defend is going to depend on what kind of tech the Protoss player has and the timing of their third gas. What kind of position you go into endgame with is entirely dependent on when Protoss can secure their third gas, and when you can secure your fourth.
Against +1 Zealot/Archon push
Lurkers are the most effective weapon against a +1 Zealot/Archon push, as it forces Protoss to wait until they get detection and possibly forcing Protoss to build Dragoons that deal less damage per second. You may consider placing more Sunken Colonies if you lack time to produce Lurkers.
Against DT/Corsair or Reaver/Corsair
Both of these are a matter of defending against the Corsairs, protecting your Overlords, and trying to build up your army without taking damage. The advantage of Corsair/Reaver and Corsair/DT is the mobility that the Corsairs give the Protoss army. A good way to nullify this advantage is to place burrowed Zerglings around the Protoss' bases - this will allow you to know whenever the Protoss moves out with his Shuttles without him knowing it. With Shuttles, both the Reaver and the Dark Templar can do ground damage almost anywhere. If you think you need it because the Corsair number is becoming alarmingly large, you should think about researching Burrow (protects your Drones too), Overlord speed (protects against Corsair harass), or using Sunken and Spore Colonies to defend your bases.
To apply pressure at this point in the game, you need to be able to set yourself up for the endgame, or, alternatively, get enough units so that you can soundly defeat the Protoss ball. Containing your opponent with Lurkers and bringing Hydras can make him feel insecure and place more Cannons. If you chose to go for Mutalisks, try to delay the Protoss push by sniping key units like Templars or Shuttles, aside from the usual Probes.
Late game starts when either the Protoss player gets their third gas, or the Zerg player techs to Hive. At this point, it is much more about base count and army composition than anything else.
Hive tech - Use Dark Swarm and Zerglings to take down undefended Protoss expansions; attempt to use your much larger mobility to attack away from the Protoss army.
Scouting and reacting to the Protoss army composition and expansion count is very important in late game ZvP. Hopefully by the late game you'll have thinned out the Protoss' Corsairs, so you can keep your excess Overlords around the map instead of herded together in your base. You should also keep small groups of Cracklings running around the map to delay Protoss expansions.
This is entirely dependent upon what the Protoss player has been doing all game. If the Protoss player has full upgrades, and is planning on going Corsair/Carrier, then you should go Guard/Devourer/Muta to counter. If the Protoss player has a large number of High Templar to go along with his Zealot and Dragoon army to counter your mid game Hydra force, then you should try to go Ultra/Ling. More often than not, when the late game rolls around the Protoss will have a solid economy, so his or her unit combinations will be able to change rather quickly, and will consist of more powerful, higher tech units than in the mid game and early game stages. Expect to see more use of Reavers, Dark Templar, and Archons.