How much spousal support will I have to pay if we share custody, or the kids choose to live with me?

Spousal support has nothing to do with shared custody, you are still responsible for spousal support based on your income.

It will be difficult for her to make up for all the years of income building, reputation building, learning, and promotions that she has missed. Getting hired when you are nearing 60 is not going to be easy for her. Child Support and Spousal Support are two separate things. Child Support is a simple calclation based on father's income, mother's income, and the amount of time spent living with each (and some other factors which impact the calculation to some degree). Spousal Support is based on the court's investigation of many, many factors in determining the standard of living during the marriage and how much the non-working parent gave up in order to raise children and provide a stable home to the working spouse, for a final order of spousal support, the Judge cannot use a computer formula, as they do for child support. Without knowing some very important details, I would estimate that if the child is shared, child support will be about $1,000 per month and spousal support will be about $2,000 per month, meaning she will have to live on only about $3,000 per month until she gets a job, at which time those amounts may change to some degree (depending on how high her earnings are), or not change at all if she still isn't up to her former standard of living with the 3 k and her own income. Spousal Support is a complicated area of law, I suggest you seek out legal assistance.

This is a complex question that is extremely fact specific. There are numerous factors involved such as your wife's age, ability to find work, the actual number of years she was out of the work force (15), her prior occupation and education, etc. So, you need to consult with an attorney and likely hire one to calculate and negotiate. Generally in a long term marriage (10 years or more) - and this is a huge generalization - you could be paying for as long as half the length of the marriage - too many other unknown factors to estimate the amount such as if you own a home, living arrangements post-separation and expenses for children, etc. You need a lawyer or you need to find self-help pro bono services that can assist you and give you a better assessment - there are books that may give you a better idea of how it will play out - I like Ed Sherman's book for laymen.

Child support is based on a comparison of your income with hers and the time share. Spousal support after that calculation occurs. (You could ask for child support.)