Generally, most states do not have a set "black and white" amount of alimony based on income and length of marriage. Rather, it is mostly a "grey" area that is often litigated in the Courts.
Because you will be paying all utilities for her, that would typically be considered a type of alimony. So the utility expense should be considered part of the alimony amount.
Of course, you will also have to support yourself as well, so you should take that into consideration too.
From a general fairness standpoint, I would suggest to you that because her rent and utilities are paid for, the rest of her expenses (for the most part) would be discretionary. Given that is the case, I would suggest that you calculate your after tax income and use that as a starting point. Let's say that on a 50,000 income, you are left with 44,000 after taxes. Then subtract your rent/mortgage and your utilities. Then subtract the amount you will be paying for her utilities. The money left over is then for discretionary expenses. I would suggest that a 50/50 split of the discretionary income would be fairly equal.
You may want to consider other factors too. Perhaps, she has some assets she can live from. Or, perhaps she is able to earn a salary. Even if she only made minimum wage, as long as she is healthy and can work and make some money, then you may want to take that out of her portion of the "50/50" split.
Keep in mind that the IRS allows an ex-spouse to claim off of your Social Security benefits for marriages over 10 years. (It will not reduce your benefits, rather it is just a benefit to the ex-spouse). So, she may not know that she is entitled to that benefit and if you inform her, that may put her at ease some.
Here is the IRS website link that explains this http://www.ssa.gov/retire2/yourdivspouse.htm
So, to sum up, there is really no amount that is set in stone. Rather, it should be fair to both you and her. If you both can agree to something that you are okay with, then that may be best. Otherwise, it can be litigated through the Courts. Of course, if the lawyers get involved, there will be less money left over for you and your (soon-to-be-ex) spouse.
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