There is no way of ever knowing exactly how long your divorce case will last, but the more willing you are to fairly negotiate issues, the faster the process will take.
Also, you will need to consider that some states have a minimum waiting period required after filing for divorce for the dissolution of marriage to be finalized.
If everything goes smoothly, you may not be looking at a long divorce case, but in most cases, the ex-wife will typically argue many points in your petition for divorce and will thus extend the period of time in which the case will be completed.
This is why it is very important that when you file the divorce petition, you also set a trial date for the case as well.
This way, you know that the case can only drag on for so long before the trial date is set to come up.
Typically though, you can expect a divorce to take up to about a year, even longer if it is a more complicated case where a large amount of assets are involved and child custody is also an issue.
The more your judge has to split, the more complicated and extended your case may be.
Either way, it's a good idea to be prepared for a long divorce case, since most ex-wives will do everything they can to stall and delay the divorce in order to get exactly what they want from you.