The biggest factor - by far - for how long your divorce will take is whether the divorce is contested or uncontested.

If you and your soon-to-be ex can reach an agreement on how everything will be divided, how custody and visitation will go for any children, and the amount of any child support or alimony, an attorney can memorialize your agreement into a legally binding form within one to two weeks. 

If you can't reach an agreement, the process naturally takes much longer.