When's the Next MacBook Line Refresh?

I've been anxiously awaiting the rumored refresh of the MacBook Pro and MacBook Air, and my 2008 MacBook is aging rapidly. Its 2.4 Ghz processor and 2GB of RAM are proving too anemic for Lion, Safari, and an IDE. Since that's what the majority of my time on a laptop is spent doing, the developing inadequacy of the MacBook is problematic.
So I want to know when I can expect to purchase a new laptop. As such, I'm going to make a couple assumptions. They are:
  1. The MBP and the MBA will be updated at the same time.
  2. This update will be happening sooner rather than later, since the machines are rumored to be in production.
Heading to the MacRumors Buyer's Guide, we see that the following are the recent lifespans for Apple's current MacBook line.
Using my assumptions, I choose to throw away any lifespans longer than 300 days for the MBP and the the MBA. That leaves the MBP with an average lifespan of 237 days, and the MBA with 261 days. We'll just use the middle of these two, and that's 249 days. As of today, the MBA is 251 days past its last update. That leaves the MBP, which is 155 days young. In the recent past, the MacBook Air was updated at 273 days. That would mean that we would see an update in three weeks. That seems like a good earliest window. The MacBook Pro would require another 80 days to reach its own average, so that would push any update out to June.
Screen Capture 2012-03-27 at 11.11.18 PM.PNG
Using my assumptions, I choose to throw away any lifespans longer than 300 days for the MBP and the the MBA. That leaves the MBP with an average lifespan of 237 days, and the MBA with 261 days. We'll just use the middle of these two, and that's 249 days. As of today, the MBA is 251 days past its last update. That leaves the MBP, which is 155 days young. In the recent past, the MacBook Air was updated at 273 days. That would mean that we would see an update in three weeks. That seems like a good earliest window. The MacBook Pro would require another 80 days to reach its own average, so that would push any update out to June.
I would not be shocked to see these machines debut with Mountain Lion, which is not due out until this summer. Summer officially begins on June 20th, and I imagine that Apple will want until at least then (knowing how most software development goes).
While I would love to be wrong, I believe that we are not going to see an update to the MBP or the MBA until summer. While that gets me a full 4 years from my MacBook, it doesn't alleviate my current performance issues.
Update: Also see Marco Arment's take using the recently released Ivy Bridge schedule. Spoiler: June looks to be exciting.