Assuming you are over 21 years of age there is no quota for parents of U.S. citizens.
Thus the only delay would be the processing times.
If your parents are currently in the U.S. and last entered legally (as tourist or in some other category) they can apply for adjustment of status. Currently in the Los Angeles area it is taking 5 to 6 months to process those types of cases (your parents can remain here while the paperwork is pending). If they are abroad it could take around a year depending on what consulate/embassy they will be applying at.
I recommend you contact a qualified immigration lawyer, such as those who volunteer to answer immigration questions for Avvo, to advise you since other factors may be involved.
Certified Immigration Law Specialist*
Law Offices of Tasoff and Tasoff
16255 Ventura Blvd. Suite 1000
Encino, California 91436
About a year
J Charles Ferrari Eng & Nishimura 213.622.2255 The statement above is general in nature and does not constitute legal advice, as not all the facts are known. You should retain an attorney to review all the facts specific to your case in order to receive advise specific to your case. The statement above does not create an attorney/client relationship. Answers on Avvo can only be general ones, as specific answers would require knowledge of all the facts. As such, they may or may not apply to the question.
If you are 21 years of age or older, your parents are considered "immediate relatives". There are not subject to any numerical quota, and can immigrate to the U.S. in a matter of months.
(213) 394-4554 x0 Mr. Shusterman is a former INS Trial Attorney (1976-82) with over 35 years of immigration experience. His response to your question is general in nature, as not all the facts are known to him. You should retain an attorney experienced in immigration law to review all the facts in your case in order to receive advice specific to your case. Mr. Shusterman's statement above does not create an attorney/client relationship.