While it is hard to give you an exact exposure period, the charges you are facing are very serious felonies. You may also be facing federal charges and state charges. You need to contact an attorney as soon as possible in order to secure your rights and start building your defense. Good luck.
You should NOT be talking to ANYONE about this case except an attorney. Avvo is a public website designed to give general information, not specific advice, and there is no way anyone can predict what kind of time you may be facing without knowing the exact charges.
There are numerous possible charges that you could face, some involving jail time, others than could land you in state prison. There is a possibility that charges could be filed in state or federal court, and that would affect the potential time, too.
One of the worst possible consequences is lifetime registration as a sex offender. If that happens, you will be prohibited from living within 2,000 feet of schools or parks -- which can mean most of an entire city could become off limits for you.
Again, don't talk to ANYONE about this situation. If the police contact you, politely say you will not answer any questions, and want to speak with a lawyer. If someone (particularly the minor) calls you on the phone to talk about the incident, there is a very good chance it's a "pretext phone call," which is set up and recorded by the cops, who are hoping that you will make incriminating statements.
If you can't afford to hire your own attorney, the judge will appoint one when you go to court.
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.