How many parts need to fail before Apple will replace a laptop??

Anecdotally, it seems that Apple has to try and fail to solve the same problem three or more times before a machine will be replaced instead of repaired. But all such decisions are made on a case-by-case basis, and there is nothing published that describes the criteria (or incidental considerations, if there are any) that feed such decisions. So only Apple can answer your question. If you aren't getting satisfaction from the Apple reps you've spoken to so far, the next level of escalation will be Customer Relations at 1-800-767-2775.

Hard drives and optical drives, being mechanical devices with high-speed moving parts, will all fail eventually and are very easily and inexpensively replaced. How long they last is highly variable. You've had three fail and be replaced under warranty, which makes you fairly lucky in the sense that those failures cost you little or nothing. There have been some problems with defective batteries, and Apple has been good about replacing them for free, sometimes even out of warranty. I've never before heard of any problem at all with an AC adapter "duck head", so that was a complete fluke for you. Drives, batteries and AC adapters are all purchased by Apple from third-party vendors, and their failures imply/suggest nothing whatsoever about the basic quality of the MBP itself. Nor have you had any basic failures of the MBP, so in my opinion you have considerably less cause for worry than you may think.

If your machine is really a year and a half old (purchased early in 2009), then its AppleCare Protection Plan coverage lasts until early 2012. If your AppleCare coverage really expires in April 2011, your machine was purchased in April 2008 and is now more than two years old.

My AppleCare expires in less than 3 weeks. In the last 2 weeks, Apple has replaced my DVD drive, my LCD panel, and my Logic board. Prior to that, just 7.5 months ago, they replaced the Logic board for another issue.

This most recent Logic board replacement was due to an error on the part of the repair tech at the Apple Store who did the first repairs 2 weeks ago. They "accidentally" replaced my hi-res (1920x1200) LCD with the "standard LCD" (1680x1050). Once they put the right LCD panel on, the Logic board would not feed enough power to it for the back lights upon resume from sleep, so they replaced the Logic board to resolve that issue.

As of right now, the only "original parts" left on my MBP are the case, keyboard, trackpad, battery, & power adapter. My 3 year old battery only has 17 cycles on it, and is reporting that it can still hold 84% of it's original capacity.

GeekBoy.from.Illinois wrote:
As of right now, the only "original parts" left on my MBP are the case, keyboard, trackpad, battery, & power adapter. My 3 year old battery only has 17 cycles on it, and is reporting that it can still hold 84% of it's original capacity.

MagSafe board, Left I/O board, HD, bottom & top case, fans, bluetooth and airport module, inverter board (if your machine is 3 years old it's most likely pre-clamshell) are all original as well. 🙂

How on earth did you manage to only get 17 cycles on a battery after three years?

And how on earth did you manage to lose 16% of your power after only 17 cycles?