Where does the 90 seconds limit for plane evacuation come from?
90 seconds comes from a safety aspect, specifically a fire safety aspect

Aviation Safety Study SA9501 Footnote 3 says:


  Internationally, industry accepts 90 seconds as a reasonable estimate of the survivable time in an evacuation where fire is present.


Which means that it is predicted than a burning aircraft is not sustainable for life beyond that period, but interestingly this is not the condition that the aircraft is certified under.

FAA Chapter 77 Section 1.7 says:


  A full-scale emergency evacuation demonstration simulates an aborted takeoff. The operator must show that the aircraft, emergency equipment, and emergency procedures allow the evacuation of the aircraft at full seating capacity, including crewmembers, in 90 seconds or less.


So the 90 second rule comes from a fire safety aspect - as humans are not expected to survive in those conditions for longer than that; but the aircraft is only tested on whether it can evacuate on an aborted takeoff (not specifying that a simulated fire must be the cause of this aborted takeoff).

Sources: https://www.skybrary.aero/bookshelf/books/2699.pdf

http://fsims.faa.gov/WDocs/8300.10%20Airworthiness%20Insp%20Handbk/Volume%202/2_077_00.htm