Frequently Asked Questions - Unlawful Presence
This is another complicated concept with some very serious consequences. Briefly, if you have been unlawfully present in USA for more than 180 days, you cannot come back to USA for 3 years and if unlawfully present for a year or more, you cannot come back for 10 years. Very generally speaking unlawful presence begins after expiration of the I-94 or after having been asked by the govt. to leave USA.