As of January 25, 2010, the vast majority of people sentenced to jail will only be required to serve 50% of their time. This is a big change from existing law, where the majority of people are required to serve 2/3's of a jail sentence.
The new rule will not apply to people with a prior conviction for a serious or violent felony, and to those who are required to register as sex offenders. Assuming you fall into neither category, you should only be required to serve 90 days of your sentence (as opposed to 120 days under existing law) assuming you don't begin serving your sentence until January 25.
I have written a much more detailed explanation about this new law on my Web site's blog. I've linked to it below.
Maximum likely time has been answered at 90 or 120 days. It is not suprising that public defenders might have different answers. Actual time frequently varies from jail to jail and county to county, based on early release and conditions related to jail crowding. Early release based upon these factors can only be estimated based upon recent experience. It is always subject to charnge.
The answer to your question depends on the date that you are sentenced. The law has changed the credits that you are entitled to receive for custodial time. The original calculation was for every two days you served, you would receive credit for one day. On 1/25/10, the calculations will be two days of credit for every four you serve.