There is no jail minimum on possession of cocaine if you are probation-eligible. The 90 days you might have heard of could be for being under the influence of cocaine (health and safety code 11550) as opposed to possession of cocaine (health and safety code 11350) It will depend on the culture of the court. Some courts are paternalistic and will try to put you in residential rehab in lieu of jail, others - especially downtown- will put you on non-reporting felony probation and time served with no other conditions just in hopes they won't see you again. Felony courts often have more pressing cases than possession of a small amount of cocaine.
If not eligible for DEJ or Prop 36 and you have to plead to the H&S 11350 felony, the judge can do anything from probation to state prison. If you were given a state prison sentence, you may serve your time in the county jail (this depends on your priors). If you're not eligible for DEJ or Prop 36, you should absolutely call a lawyer who is familiar with the courthouse in which your case is heard to get you the best possible disposition or take your case to trial.
Attorney answers to questions on Avvo.com are for general purposes only and do not establish an attorney-client relationship.
If you are granted probation, there is no minimum amount of jail time for possession of cocaine.
If the judge denies probation, you could receive up to three years. That time would be served in a county jail facility UNLESS you have a prior conviction for a serious or violent felony under the Three Strikes law, or you are a registered sex offender; if either of those applies, you would have to serve a sentence in state prison.
However, I am concerned that you don't qualify for probation under Proposition 36/Penal Code 1210, or diversion under Penal Code 1000. That indicates that you must have some sort of prior conviction, or another charge in the same case, which puts you in a more difficult sentencing position.
Bottom line: in order to give you a meaningful answer, your lawyer has to have all the facts. If you can't afford to hire an attorney, the judge will appoint the public defender to represent you.
Please understand that this is a general discussion of legal principles by a California lawyer and does not create an attorney/client relationship. It's impossible to give detailed, accurate advice based on a few sentences on a website (and you shouldn't provide too much specific information about your legal matter on a public forum like this site, anyway). You should always seek advice from an attorney licensed in your jurisdiction who can give you an informed opinion after reviewing all of the relevant information.