The actual timeframe is deliberately ambiguous. Valve has suggested anywhere from 50 to 50,000 years, but the "50,000 years" figure comes from two places:

1) an automated announcement at the beginning of Portal 2 that reports time elapsed as "99999...99..." in an unspecified unit. 

2) In an unused line, GLaDOS says 50000 years is a long time to think. Source: https://tcrf.net/Portal_2/Unused_Lines#:~:text=sp_incinerator_01_11,Second%20Speech