It depends on whether he will be able to adjust status or will have to move forward with consular processing. Either way you may want to speak with an immigration attorney to way your options. Make sure he will be admissible on all grounds should he have to leave the country to move forward with consular processing. Adjustments in my area are taking around 4-5 months from the time the adjustment package is sent in to the interview. Consular processing will take longer.
The process can take about one year if you are diligent. If he entered illegally then he will have to leave the US but only after he has applied and been approved for a waiver.
The answer provided here is general in nature and does not take into account other factors that may need to be reviewed for a more precise answer. You should consult with an immigration attorney before taking any action. The answer here is not intended to create an attorney-client relationship.
If he came illegally, and is not otherwise inadmissible (crime, public charge, fraud etc), and he is eligible by virtue of 245i, then yes, the process could be fast, less than 6 months. But the critical thing there is eligibility through 245i, which only a few people qualify. Otherwise, he'd have to go through the I-130 / provisional waiver route, which requires a showing of extreme hardship to his USC spouse, and eventually he'd have to leave the US.
The 3-4 month time frame you are referring to applies if he is eligible to receive his green card in the U.S. without having to leave the country. In your husband's case, you said he entered illegally / undocumented. As a result, assuming there are no other issues of inadmissibility (criminal violations, previously issued removal order, multiple re-entries into the U.S., etc.), he may be required to depart the U.S. so that he can consular process.
Once you file the initial petition, and once you are approved, your case will be sent to the National Visa Center. At that point, you can determine whether he qualifies for the new unlawful presence provisional waiver. This new program recently established will allow him to be approved before he departs the U.S., thereby decreasing the amount of time you and him are physically separated.
It is best to retain an immigration lawyer as soon as possible.
Lena Korial-Yonan, Esq.