it seems that IBIS check takes only a day or two.
IBIS Procedure:
INS performs background checks using the "IBIS" database for all types of applications and petitions, whether filed at the regional Service Centers or the local District Offices. These checks occur within 15 days of receiving the case and remain valid for 35 days. If the adjudication occurs more than 35 days after the IBIS check, a new check is required.
These timeframes can become an issue for Adjustment of Status (I-485) applications. In those cases, the Service Center sends an approval notice directing the applicant to the Local District Office for a passport stamp. If the passport stamp is done within 35 days of the last IBIS check, no additional check is needed. If a further IBIS check is needed before the passport can be stamped, it may take only a day at many of the local offices. However, if the check results in a "hit" (a match with a record on file of a person with a criminal issue or other serious problem) the case may take much longer to resolve.
Note that, since IBIS is a name check system, if one has a very common name there may be a "hit" based on the record of some other person who happens to have the same name. In the event of a hit, the local office is required to place a stamp in the passport as evidence of permanent resident status, valid for 30 days only. This stamp will provide evidence of authorization to work and will allow travel for the 30-day period. No mention was made regarding procedures if the matter is not cleared up within 30 days but, presumably, the person could obtain another stamp if needed.