From "The Economy" section of the
Game Manual:
If you go into debt, you will be charged a 5% penalty each week until you are positive again at the time of a economic update. If your account is below -$500,000 at the time of a weekly economic update, you are given two weeks to bring your account back above -$500,000.
In other words, as soon as you get into the negatives (ex: -$1).
If you'd like a more detailed explanation, I suggest you head over to the "Economy" section of the
Game Manual.