A minimum of ten days if convicted of the second DUI. However, since he was still on probation for the first one, the State is likely to seek more. A second DUI conviction within five years of a prior carries a maximum of up to 270 days in jail. If he took the breath test and his BAC was .15 or higher, the maximum penalty increases to 364 days in jail.
He also is subject to a violation of probation which could subject him to between 6 months and 1 year in jail for just the violation. The facts of the first case would determine the amount of time he is facing. If there was an accident it could be upto one year.
Once they violate his probation, depending on the Judge, he will be taken into custody until he resloves both cases. Contact his attorney from the first case or an expeirenced local DUI attorney for more complete answers to your question.
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