Because the Aperture Science Announcement System at the very beginning of the game says, "you have been sleeping for nine, nine, nine, nine, nine-"
In a official announcement from Valve, they stated that it was hundreds of years, and that measurement was probably a malfunction from the Aperture Science PA.
(pulled from trivia on chell's page)
- It's not exactly clear how much time Chell has spent in stasis between events of Portal and Portal 2. At the beginning of Portal 2 she's woken up first time after 50 days of sleep, for mandatory set of physical and mental exercises. Next time she wakes up, the announcer fades out (or stutters) after uttering 5 nines, which could either mean that Chell was sleeping for at least 273 years or that the announcement system reached a maximum. It may alternatively have broken after much of the facility fell into disrepair after the defeat of GLaDOS at the end of Portal which could also mean that she might have been sleeping for less time than announced.
99999 days is roughly 273.78 years, so presumably the clock monitoring chells pod failed when it hit its 100000 day timer.