Good evening, and thanks for your reply.
If you'll tell me what state you're located in, I can give you specifics regarding the residency requirements for that state.
You can hire an attorney who specializes in international divorce laws, but, unless your divorce has complicated international issues (for example, property in both countries that will have to be divided between you and your wife, custody matters -- although you've said your son is 20 years old, which is past the age of majority, and therefore not a concern with respect to custody and child support, etc.), I believe a local family law attorney should be able to help you. I'm sure the international family law attorneys charge a pretty penny for their services, and for an "ordinary" divorce with no major issues to be settled, I really don't think a specialist would be necessary. But, that's up to you, and which type of attorney you would feel more comfortable with.
To serve the divorce petition, you can use an international process server (there are companies that specialize in this), or, the court will likely be able to serve the petition on your wife via certified mail, FedEx, DHL, or a similar service. The court simply needs to be assured that your wife has received a copy of the summons and divorce petition in order for the divorce to proceed.
Many states (including my own state of West Virginia) nowadays have their family law forms available for download on the Internet. Your state may likewise have them available on the Internet. Once you've told me what state you're in, I'll be able to tell you more about this.
At any rate, the first thing you'll need to do to initiate the divorce process is to file your divorce petition with the clerk of court in the county where you reside. Next, your wife will be served with the petition and notice that she will have a certain amount of time (usually 20 or 30 days, again, depending on your state law) to file an answer to the petition. At some point after that, the case will be set for final hearing. There is really no "average" amount of time that it takes for a divorce action to proceed from start to finish. If your wife is served quickly, and she is agreeable to getting the divorce, property division, etc. I would predict that everything could be completed in a matter of a few months.