1) 180天，以I-485 receipt date算起。白纸黑字在USCIS原文里;
2) 180天的定义是指180天USCIS还没批I-485， 跟AOS申请人一点关系一点都没有。理论上讲，AOS申请人，如果能预测I-485在180天以内不会被批准，拿到receipt就可以换工作。
换工作， layoff等等:
a) I-140已经被批准， 180天以后，找到一份和I-140相近(deion & pay)的工作，原公司撤除I-140 sponsorship也不会影响I-485
b) I-140已经批准， 180天以内换工作，原公司没有撤销I-140: 超过180天后，要通知USCIS AC21以确保万一
c) I-140已经批准， 180天以内换工作，原公司撤销I-140: 还在研究。网上律师说法不同。
d) I-140还没批准， 180天以内换工作，I-485会被拒， 不能用AC-21
20.2 Petition Validity.
(a) General . Immigrant visa petitions are valid indefinitely until they are used as a vehicle for immigration or adjustment of status or until they are revoked. In specific cases, an approved petition may be “converted” to another classification. For detailed information on such cases, confer with applicable regulations in 8 CFR 204. In any instance where there is a significant lapse of time since the petition was approved, the adjudicator considering an application for adjustment (or a consular officer handling t he immigrant visa case) should take appropriate steps to ensure the relationship, job offer, etc. on which the original approval was premised continues to exist. Occasionally, USCIS will receive such a petition back from a consular office with a request for follow-up action to reaffirm the facts of the petition. Such cases should be handled routinely, verifying the facts in the same manner as if a new petition were being considered.
(b) Approval of a Subsequent Petition . At times, a petitioner may resubmit a petition seeking the same benefit as the prior petition, although the earlier petition may remain valid. If such a petition is approvable, the remarks block of the petition should be noted to reflect the filing and approval dates of the first petition. The original priority date is assigned to the new petition.
(c) Validity after Revocation or Withdrawal . Pursuant to the provisions of section 106(c) of the American Competitiveness in the Twenty-First Century Act (AC21), Public Law 106-313, the approval of a Form I-140 employment-based (EB) immigrant petition shall remain valid when an alien changes jobs, if:
· A Form I-485, Application to Adjust Status, on the basis of the EB immigrant petition has been filed and remained unadjudicated for 180 days or more; and
· The new job is in the same or similar occupational classification as the job for which the certification or approval was initially made.
If the Form I-140 has been approved and the Form I-485 has been filed and remained unadjudicated for 180 days or more (as measured from the form I-485 receipt date), the approved Form I-140 will remain valid even if the alien changes jobs or employers as long as the new offer of employment is in the same or similar occupation. If the Form I-485 has been pending for less than 180 days, then the approved Form I-140 shall not remain valid with respect to a new offer of employment.
Accordingly, if the employer withdraws the approved Form I-140 on or after the date that the Form I-485 has been pending 180 days, the approved Form I-140 shall remain valid under the provisions of §106(c) of AC21. It is expected that the alien will have submitted evidence to the office having jurisdiction over the pending Form I-485 that the new offer of employment is in the same or similar occupational classification as the offer of employment for which the petition was filed. Accordingly, if the underl ying approved Form I-140 is withdrawn, and the alien has not submitted evidence of a new qualifying offer of employment, the adjudicating officer must issue a Notice of Intent to Deny the pending Form I-485. See 8 CFR 103.2(b)(16)(i). If the evidence of a new qualifying offer of employment submitted in response to the Notice of Intent to Deny is timely filed and it appears that the alien has a new offer of employment in the same or similar occupation, the USCIS may consider the approved Form I-140 to remain valid with respect to the new offer of employment and may continue regular processing of the Form I-485. If the applicant responds to the Notice of Intent to Deny, but has not established that the new offer o f employment is in the same or similar occupation, the adjudicating officer may immediately deny the Form I-485. If the alien does not respond or fails to timely respond to the Notice of Intent to Deny, the adjudicating officer may immediately deny the Form I-485.
If approval of the Form I-140 is revoked or the Form I-140 is withdrawn before the alien’s Form I-485 has been pending 180 days, the approved Form I-140 is no longer valid with respect to a new offer of employment and the Form I-485 may be denied. If at any time the USCIS revokes approval of the Form I-140 based on fraud, the alien will not be eligible for the job flexibility provisions of §106(c) of AC21 and the adjudicating officer may, in his or her discretion, deny the attached Form I-485 immediately. In all cases an offer of employment must have been bona fide, and the employer must have had the intent, at the time the Form I-140 was approved, to employ the beneficiary upon adjustment. It should be noted that there is no requirement in statute or regulations that a beneficiary of a Form I-140 actually be in the underlying employment until permanent residence is authorized. Therefore, it is possible for an alien to qualify for the provisions of §106(c) of AC21 even if he or she has never been employed by the prior petitioning employer or the subsequent employer under section 204(j) of the Act.