After an immigration arrest, how soon will the undocumented immigrant be deported?

Even if it is true that your brother has no choice but to leave the U.S., how soon this happens depends on a number of factors, such as:

• Whether he has been ordered removed before. If so, the immigration authorities can simply act on that previous order of removal and send him back to El Salvador right away.
• How long he has been in the United States. If it is for fewer than two years, he may be subject to “expedited removal,” meaning that he can be deported very quickly without a hearing before an immigration judge. If, on the other hand, he has been in the U.S. for ten or more years, has been of good moral character, and can show that his deportation would cause “exceptional and extremely unusual hardship” to a qualifying relative(s) who is (or are) U.S. citizens or lawful permanent residents, he may qualify for “cancellation of removal” or a green card.
• How far from the U.S. border he was when picked up. If he was within 100 miles, and cannot show that he entered the U.S. more than less than 14 days before the arrest date, he may also be subject to expedited removal, and will not receive a hearing.
• Whether he fears persecution or torture in El Salvador. If so, he might avoid the effects of expedited removal, and be able to present a case for asylum, withholding of removal, or Convention Against Torture relief before an immigration judge. See “Asylum & Refugee Status” for more information on this.
• Whether he would prefer to leave voluntarily, without the hassle of further proceedings. See “Voluntary Departure: Who Is Eligible?” for more information on this option, which avoids the effects of a removal order, such as a long-term bar to future U.S. immigration.
• Whether he is placed into removal proceedings before an immigration judge. If so, see “Judge's Decision in Immigration Court: How Long It Will Take to Get” for further analysis of the likely timing.

As you can see, dealing with deportation proceedings is a highly complex area of immigration law, and getting your brother in contact with an experienced immigration attorney would be well worth your while.