How early should I build a defensive item as a mid laner?

In general, you should get defensive items as needed. Assuming you are playing a fairly typical mid hero, you are usually going to be focused on damage instead of survivability. Your decision will change based on the individual game. If you are beating your lane opponent, you should probably go for more damage (or attack speed, ability power, etc.) for the added gank/killing power.

If you are being beaten inlane, you should almost always get an early defensive item. Avoiding death is very important to prevent snowballing. An example would be an AP champ losing to a Zed. An early Seeker's Armguard will hurt your early game damage a lot, but you will make up for that by avoiding death early on which will make your late game much better.

Yasuo is a more specific champ because the usual build (in my league at least) is a glass cannon build. Still, if you are losing your lane, a basic defensive item (under 500 gold) will help you in the late game. For most mid champs that would be fine but Yasuo has a fairly weak early game compared to many midlaners so it may be better in the end to accept your weakness and go straight for a few items so you can get to mid/late game faster.

Spectre's Cowl may have been too expensive of an item, unless you play Yasuo as more of a bruiser type champ. Even if your goal is tanky with good damage, you probably could make do with just a small piece of a defensive item until your dps is better.

There is no real rule of thumb. This decision depends on many factors:

• How good can i play my champ?
• Is the enemy able to oneshot me? (Ahri, Zed, LeBlanc)
• Does my champ offer anything if built more tanky? (Utility or insane scaling/ high DoT?)
• Does the defensive item I'm about to built make sense?

In the following, I'll try to answer all of these questions:

How good can i play my champ?

This is the most important aspect. You always have to keep in mind: Tankiness forgives mistakes, damage doesn't! If you are not that comfortable on your champ, you will most likely do a few mistakes which the enemy midlaner can punish heavily and set you back even further. On the other hand, more tankiness means less dmg, which means you will have to play around that factor.

Is the enemy able to oneshot me?

I think we all faced that situation at least once. You're happily farming your lane, until ...

Zed used death mark. 
It's very effective.
Whoever you plays fainted.

This is an extremely frustrating and also extremely dangerous situation. And there is also only one answer if you play an AP midlaner: Zhonyas Hourglass. This item offers a few advantages:

• Good amount of AP
• Protection from oneshot Champs or heavy AoE's
• Good amount of armor (poke/all-in protection)
• Mental aspect (a decent zed will not waste your ult on you if he knows you got a zhonyas, so you will be save from him)

As a AD Midlaner, you have QSS. But you shouldn't rush this since this only gives you defensive stats, which will set you back too, maybe even more than dying once or twice.

Does my champ offer anything if built more tanky?

This is also a pretty important aspect when thinking about defensive items early. Do you have enough impact with your basic scaling/utility to have any impact in a teamfight, even with less damage? A good example here is lulu. Even if you - somehow - manage it to completely fuck up the laning phase, you can build some tankiness and still have a massive impact in fights with your slows, shields, polymorphs and knockups. You can protect important members of your team and help them in that way. A Zed can't built tanky. Neither can a katarina. Rule of thumb: Mages can be built tanky, assassins can't (Except for ahri)

Does the defensive item I'm about to built make sense?

Yeah, you play Orianna and get stomped by a LeBlanc. Oh nice, Abyssal Scepter, AP and MR, nice, bought it - nope. Also, your choice with Spectres on Yasuo wasn't very smart since this item builts into nothing valuable for him. You can buy the Spirit Visage, yes, but that's not an item yasuo needs. The correct choice in your situation would have been hexdrinker.

Now i wanted to list the best defensive items, but that's already explained in the post of Jutschge.

When you play at middle lane your team is expecting decent damage output from you. However in some cases you may start with negatron cloack to build abyssal after. Good example when you matched as melee champion like Katarina against burst champion like Le Blanc.