(Not something that grinds my gears. Just some fan hypothesizing.)
At the beginning of Portal 2, the computer voice first tells the player that "you have been in suspension for fifty days" (with the "fifty" being said with a completely different, automatic-sounding voice). Then later it says "you have been in suspension for nine... nine... nine... nine... nine..." etc.
Every single fan theory I have seen always speculates how long the time has been between the two events. They speculate that it has been 9999999 days, or hours, or whatever.
That makes absolutely no sense. It's quite clear that a) an algorithm is (in-universe) used to pronounce the number, and b) said algorithm was malfunctioning because of the state of decay of the facility, trying to pronounce some number starting with 9, but ending up in an endless loop with that first digit, like a broken record.
Otherwise it wouldn't make any sense why it first says properly "fifty" (rather than "five zero") but then "nine, nine, nine...". If it had been trying to say a large number, it would have said eg. "nine million, nine hundred ninety-nine thousand..." etc. (It also wouldn't make any sense for it to use days in the first announcement, and then eg. hours.) It's rather obvious that it simply got in a loop when trying to say the first digit of the actual number. Said number could just as well have been, for example, 900 days, or 9000 days. Further evidence is that the loop slows down by the end, ie. the computer was getting more and more corrupted by the second.
(Yes, I know that some background development material mentions tens of thousands of years, but as long as it doesn't appear in the game proper, I don't consider it canon. It's quite clear that the voice recording was intended to sound like a broken record, rather than just pronouncing a large number full of nines.)
It wouldn't make any kind of sense for the time to have been tens of thousands of years. No technology lasts for that long, and no living being, no matter how much "in suspension" they might be. And the bed, bedsheets and furniture would have turned to dust by that time. (And the potatoes of the "bring your daughter to work" day wouldn't be there after tens of thousands of years.)
The facility was overgrown with vegetation. The facility had to be so deteriorated that it was crumbling and allowed vegetation to invade it. I don't think a few years would be enough for that, which is why 900 days (about 2.4 years) might be too little. My personal choice of the actual time passed is 9000 days (plus whatever the rest of the number is, which we don't hear). That's about 25 years. That ought to be enough for the condition we see the facility in. (Although, admittedly, the potatoes contradict even this. But perhaps we can safely ignore the potatoes. Or perhaps they were preserved by irradiation or something.)
On a completely different tangent, I have a radical hypothesis: The universe we see in the Portal games is not actually the same universe as we see in the Half-Life games. They are completely separate universes.
There is a Black Mesa in the Portal universe, but it's a different one from what we see in the Half-Life games. Also there is an Aperture Science in the Half-Life universe, but it's not the same as the one we see in the Portal games.
Why do I think this? Well, consider that in Portal 2 it's established that both Aperture and Black Mesa competed over a governmental contract, and the government chose Black Mesa.
However, that makes no sense if this were a single universe. Aperture had working energy-efficient light-weight portable and weaponizable teleporting technology, and had had it for decades. Black Mesa, on the other hand, was barely at the beginnings of experimenting with teleportation technology, using hangar-sized machines which were highly experimental, and could barely form a teleport to some other universe, and which was so experimental and unreliable that it effectively caused the end of the world when it went out of control. And this was when the events of Half-Life happened (which is quite strongly implied to have happened well after this government contract was made, which means that Black Mesa's technology was even more primitive at that time, while Aperture already had had working teleportation technology for years, even decades.)
It would make more sense that the Black Mesa of the Portal universe (ie. the Black Mesa we have never seen) was even more advanced than Aperture Science was, which is why the government chose the former. Conversely in the Half-Life universe Aperture was less advanced than Black Mesa (and significantly less advanced than the Aperture we see in the Portal games.)
Note that the Perpetual Testing Initiative establishes that there are multiple parallel universes, many of them with their own versions of Aperture Science and even Cave Johnson (who in some of those universes even had a different assistant, a guy named Greg.) The Half-Life series is depicting the Black Mesa of one of those parallel universes.