If you think of it this way, when a digital clock is unplugged and plugged back -- it will flash '12:00' indefinitely until its reset, A few dozen years is all it takes to wear down wood, and upholstery from under use and bad conditions. My answer is that, Aperture Science suffered a catastrophic power failure and reverted to a backup generator, which in turn borked the system clock, and woke up all of the... still alive test subjects. Since we know that Rattman is still alive, it seriously could not have been that long. Rattman has been around from Portal 1 to Portal 2. We see him in the ending credits, along with evidence of him living in all of his dens throughout the game. If it has truly been longer than 60 years, Rattman would have died, and all of his murals would have been more decrepit and grown over. Could be wrong, but I'm pretty sure humans don't live longer than 114 years Doug would be past his due point, and would be too old to move at all if so much time has elapsed (since we someone sprinting on the elevator ride up from GLaDOS' Chamber, and there only being two known living humans in Aperture Science that we know of.) I Estimate 30-40 years, enough time if we talk about plant growth along with possible radiation leakage (if there is a nuclear reactor hidden somewhere)and Chells' science experiment, that could explain the overgrowth. Machines also degrade quickly with out power, and maintenance. No power, no central intelligence, no nanobots (jerry and his crew) All personality constructs working at low voltage levels, The breaker room underneath GLaDOS's chamber looks almost untouched and not corroded (Also Switched off). Can't have been too long, or you wouldn't be able to flip any of the breakers to turn anything on, or to divert power. (better question-- WHY WERE ALL THE BREAKERS SWITCHED OFF?!)