How much time do I have to recharge the PS Vita's battery before I lose my progress?

The PS Vita has a very neat feature to avoid data loss due to a drained battery.

When its battery level reaches a critical low, it'll forcefully enter a low-power state, during which time it'll appear to be in standby, but cannot be awakened from it until its battery has surpassed a certain treshold.

Once the battery has been recharged, the Vita can be awakened as usual. Unsaved progress is not lost. This has happened twice to me thus far; the longest my Vita had to spend without power was about 16 hours.

I was wondering just how long I can leave my PS Vita in this low power state, until my progress is definitely lost. Or does the Vita maybe save a RAM image on its memory card, similarly to how a PC handles hibernation?

Knowing this will help me sleep better, whenever I forget taking the proprietary charger with me when travelling overseas.

This is something that I too myself have wondered about. Since my Vita is still under warranty I decided to chat with Playstation Support, and try and get a semi-official response.

What follows is the related parts of my chat conversation. (I have removed names to protect the innocent.)

Me: When the PS Vita battery gets low enough, the system shuts down into low power mode. How long will the system be able to last like this?
Sony Rep: Allow me a moment to look for this.
Me: Okay.
Sony Rep: The time of low battery mode could vary of the activity of the console.
Sony Rep: But as max it would be around 20-30 minutes.

Now, I hate to say it, but that is more or less a down right lie, since I've personally left my Vita in low power mode for several hours, and I don't doubt your 16 hour claim. I hate to say it, but most likely even Sony doesn't know (probably didn't care to measure it) and the rep gave me an extremely (and I mean extremely) conservative estimate to prevent a lawsuit. Business is business I guess.

However perhaps the answer is in the question. Your claim of 16 hours doesn't seem strange to me at all since I have actually left a PSP (which also features a auto-shutdown-into-low-power-mode) with an almost dead battery for more than one day (26-28 hours of the top of my head) and it survived on me. Assuming the Vita has a battery that is as good as, if not better than, the PSP then I'd shoot for at least 20 hours, assuming you don't touch the thing.

All of this said, there is no reason to try to push it. When your Vita powers down, it means get to a chager ASAP. Leave the benchmarking to Underwriters Laboratories.

As for the second part of your question, I also asked the (extremely "savvy") customer service rep about how the Vita handles complete power loss:

Me: Also, should the battery run completely dry, would data be lost? or would it always resume from where it was left?
Sony Rep: If the battery is completely drained out, then the data won't be lost, but if you were playing a game, then you will start from your last saved data.
Sony Rep: If the PS Vita battery has been completely drained, you will need to charge the PS Vita through the AC power (wall socket) and let the PS Vita charge for at least 15-20 minutes before you can turn on the system and play as you charge.

Okay, I can live with that. Basically you'll lose whatever your doing, but nothing more than that. I also know for a fact that the PS Vita implements measures to prevent data corruption from abrupt interruptions (e.g. closing apps suddenly, or not so sudden power loss.) This is well documented in the Playstation Developer manuals, but the point here is that you probably won't screw up your 150 hour save game, but you'll lose that last 30+ kill streak, which really sucks. The only reason I even mention this is because I actually corrupted a save for Afterburner on PSP doing this, but that's another story.

All in all, I can't help but feel like the more we know, the less we can be sure about. At least you can't say we didn't try.

I hope this helps! :)