Build orders: What is the timeline & how much can each base support?

So there is no good answer to this question, or it would already appear in the larger build order questions on this site. Unlike the early game (which is well defined) mid to late game doesn't have an easy answer, and anyone who gives you a BO past 20 is a snake oil sales man.

Even in the Zerg question you linked the answer was: What ever feels best.

Let's look at some of the basic Protoss builds and how they progress:

4 Gate

Usually this build is very all-in, and you're banking on winning early. However, if you do enough economic damage with a 4 Gate, but don't win you can still be at an advantage. At this point if you pressure more you're going to end up against a higher tech opposing force and probably lose, so you can expand here and maybe add on a Robo or two more Gateways.

3 Gate

One of the great things people like to do with 3 Gate is provide pressure so that they can expand. This sets them up with a nice little force as they transition into the mid game. Once expanding there are some people who like to progress to 5 Gate + Robo, or 3 Gate + Double Robo, or even (recently) 3 Gate + Double Starport. There is even a nice 4 Gate + Robo to provide pressure so that Protoss can take a third expansion.

You probably notice a number of trends between these two builds, even though I picked two builds that could not be more different there were a number of important ideas:

• Your Production must be proportional to your Income. If you have less production than your income you need to have a plan for it (eg. setting up to expand). If you have more production than your income you need to be aggressive
• Don't over commit. When you're ahead, your number 1 priority should be to get more ahead (ie. expand)
• While you always need to tech (ie. Red Queen Hypothesis) teching takes time, so don't forget that in your calculations.

Now you'll notice that a lot of what's being discussed is Income vs Production and there are a couple tools you can use to help you out:

• Protoss Calculator
• Terran Calculator
• Zerg Calculator

Remember: To get good at Starcraft you need to get good at Execution and Decision Making, not get good at following a road map.

Have you seen this wiki?

There is a ton of information that you might find useful.
Most of the build orders listed there explain the goal, and how to transition to different tech trees, among other things.