We are not attorneys and do not offer legal advice. If you are facing criminal charges we suggest you talk to a professional attorney.
According to interviews from ex-inmates at the Los Angeles County Jail it is very uncommon for a person to do their full sentence in the jail. One interviewee was sentenced to a year for Assault with a Deadly Weapon and served just 37 days before being released. He warns, however, that serving just 10% of your time is uncommon if you have violence on your record.
The rumor is that if you have done state time (time in prison) you will do 50% of your time in LA. If you haven't you should do 10-25% (depending on the current level of bookings in the jail).
Time off for good behavior isn't really what it sounds like - you don't have to do anything to get it. When you go to the jail they will most likely give you time off. In order to get it taken away you usually have to do something pretty bad. Because you do so much less time with time off in LA it is in your best interest to be on your best behavior.