Spousal Support for a "long-term" marriage (which pursuant to Family Code Section 4336 would generally be a marriage of 10 year of more) is ordinarily awarded until the recipient's death or remarriage. Spousal Support for marriages under 10 years of duration is ordinarily awarded for 1/2 of the term of the marriage. Other factors may influence the duration of Spousal Support, so you should consult with a Family Law Attorney, preferably a Certified Family Law Specialist, to discuss the particulars of your situation.
Nothing is forever. As Mr. Conviser has stated, there is a difference in the treatment of support in a "long" term marriage (one over 10 years duration) and a marriage less than 10 years. There are many, many factors a court will consider when determining a spousal support request. Those factors are found in Family Code Section 4320.
Now, after 20 years of marriage, if your former spouse does not remarry and does not move in with a male friend, you could be paying support for a long time. If the judgment has not yet been entered, you may want to ask the court to give a "Gavron" warning. What that means is the Court will admonish your former spouse that it is the goal of the State that all parties become self supporting. Basically, it's a warning that she cannot simply rely on you to pay her support and not try to make a living on her own.
You may want to consult with a family law lawyer in your area for further explanation or if you have more questions. Good luck!
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In addition to the other answers I want to add that there is nothing preventing you from negotiating an unmodifiable termination date if you and your spouse carefully bargain for it and the court is satisfied that the agreement is not unconscionable for either party or risky for the public fiscus.
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