Time is relative, this is not a new concept. In the game of Portal, it is possibly even more relative than one would believe, as it is only what you make of it.
In Portal, there are no clocks, there is no sun, there is nothing that tells you that time is even a reality (until the end of the game). This would be a great way to have the player insert their own idea of time and how it reacts with the environment, but I think otherwise. I think that time in Portal is specifically used to isolate you further, and not only immerse you in the game, but it is used to make you think about relativity.
Portal is a physics based game, one that highly relies on the player understanding their physical surroundings and how they work both within the game and the outside world. The game also forces the player to understand how to break those conventions of their surroundings. Clearly, no person can simply create portals and move from one part of the room to the exact opposite in less than seconds. This mode of transportation is not unlike teleportation, even if it does require certain aspects (portal gel, and gun) to get it’s main purpose completed. This breaking of the laws of physics we currently adhere to is quite common in Portal (and in fact the point of the game), and when you take a look to the type of time that they use in Portal, it is also very much the same type of breaking of the rules.
Time in Portal is relative to the player; if you move to place A, B will happen, but until then you can very well be occupying the same time frame for as long as you want. This is most apparent to the end boss fights in Portal, specifically Portal 2, and their attempt to break time in order to help you beat the boss. In Portal 2, time is re-set to a set point every time you attach a corrupted core. In Portal you can understand your surroundings for a time before you execute an event trigger to commence the neurotoxin. Time is entirely up to the player to use and abuse as needed, as are all other aspects of the game. The game is set to show the player that they can indeed not only create their relative rules for playing their own games of life, but they can also relatively understand their own times in a different way.
Time is something that often affects the way that we look at our future or our past, but by isolating us from the knowledge of time, we can determine how much we are actually defined by time. Chell may have very well been just some other test subject that happened to have been the only survivor, but her past history with the lab is what makes her story so compelling. The future life she has, or believes that she will have, is all that may be compelling her, and you.