My wife left me after 20 yrs of marriage for another man, the children are on their own do I have to pay alimony and for how long?

Depends on your State and what your joint assets are, how much she contributed to the marriage and several other factors.

You need to consult with an Attorney - there is an advantage to being the Plaintiff instead of the Defendant.

In most states in the US you have to pay temporary spousal support if the spouse didn't work. It's possible that she feels bad about leaving you and may not pursue any money.

You really need to contact an attorney ASAP to file and get legal guidance.

It along with how much of the value of your house, car and bank accounts you get will all have to be worked out in court.

Any alimony she would be awarded by the Court would only be temporary anyway. Alimony is usually awarded if the wife stayed home and did not work outside the home. The alimony would be basically for rehabilitation to help her get schooling to be able to support herself in whatever career or job she thinks she can handle.

You need to consult with a good divorce attorney who can guide you through this as well as give you a good idea of just how much you will be ordered to pay by the Court. Each state is different. Your financial picture dictates what the bottom monthly number will be. You could also opt for a lump sum alimony award if she'll go along with it.