There is no specific time limit as long as the Trustee is performing his/her duties. Why is the Trustee keeping the case open? That's the big question. Is he seeking to administer assets? Trying to get additional information from you? If it's simply a question of administering assets, you can always file a Motion to Abandon the assets to compel the Trustee to either get moving to liquidate them or give them up. Without more information on the specifics of your situation, I can't advise as to the best course to take. You can try contacting the US Trustee's Office and speak with the supervising attorney for your case Trustee and they may be able to put pressure where it is needed.
You should call the bankruptcy court and ask for the case administrator who is handling your case. Ask him/her why you have not received a discharge. It is possible that your case fell through the cracks. It is possible that you have not completed the debtor education course. If you can't find out from that person, contact the trustee's office and ask why they are keeping the case open so long. There must be a reason. Once you know you can try to deal with it. Good luck.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
Are you represented by an attorney? If so, have the attorney contact the court. If not, you can contact the court. If the trustee issued a no distribution report, there may be a reason why your case is still open (a reaffirmation agreement could be outstanding or your financial management certificate might not have been properly filed). It is also possible that your discharge has not been issued because of an oversight. It may also be possible that a motion or investigation is pending.
Call the court and you will know for certain. The case administrator can give you an idea. You may need an attorney if there are open issues with your case.