There really isn't a "fair". Spousal support is decided on a case by case basis based on many facts. If you both can't agree regarding what is fair, you will each need to argue why your number is a better choice.
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Unfortunately in marriage of more than 10 years (long term marriages), there is no set answers as far as what is fair as far as duration.
For long term marriages, Courts usually do not give a termination date unless there is a clear indication that at some point in the future the supported spouse will no longer have a need for support.
For marriage of less than 10 years, Judges frequently use one-half the duration of marriage.
The main issue is what are her needs and will they be met at some point. Can her monthly needs be met with disability payments and is there any possibility she could return to work at any point?
These are issues you have to negotiate together about.
Given that you were married for long term period and conceivably could be on the hook for support for a really long time, I suggest you consult with a skilled attorney regarding this matter.
-- Note that you may want to ask this attorney about your date of separation and if you could argue that your date of separation was when you both moved into separate rooms. He/she will need to know more facts about this matter.
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