Whether or not there is such a thing as a legal separation will depend upon your state. The amount of time it takes to physically separate is a manner of how quickly you and your spouse choose to do so. The legal process, when available, is likely to take at least a couple of months to complete.
Divorce similarly varies by state, and also by complexity. States typically have waiting periods for divorce, with longer periods when children are involved. It might be possible to resolve a simple, uncontested divorce with no children within the space of a couple of months. When the case is contested, or when children are involved, it's more likely to take between six months and a year -- perhaps longer.
In any case, even when the parties are in agreement as to the outocme, the process can be made longer based upon party errors, a lack of due diligence, or the court's docket being crowded resulting in delays before court hearings can be scheduled.

