Odds of Hanging On
First, the disclaimer: Promotions and individual circumstances will affect your assignment progression. Now, let's get to the nuts and bolts: After your initial training, you'll typically spend six to eight years working in a field office. For your first assignment, you could be sent anywhere in the United States, so you might not want to purchase that frequent-latte card just yet. Once you survive that, you jump into a protective assignment for another three to five years. Then you'll go back into the field, or perhaps migrate to a training or headquarters office, or to another plum DC-area position. You might rack up some impressive air mileage.
If you've got foreign language fluency, you might snag a spot in a Secret Service international bureau. Your foreign language proficiency will enable you to understand your host country's law enforcement guys, which is helpful since you've got to work closely with them.
Of course, these assorted scenarios assume you get through your assignments in one piece; that is, you don't get injured or whacked. You'll get lots of training, equipment, protocols, and procedures. However, you won't get any guarantees.