That depends on the contract the agent offers. And good agents always offer contracts.
If you have limited publishing experience but an agent takes a chance on you, the agreement can be that the agent will represent you for a period of one year or as long as two. The agent may have doubts about whether she can sell your book. If she hasn't sold it within the time limit, you're now free to try another agent.
If the agent sells the book within that time frame and wants to represent you, she will usually become your representative until one of you severs the relationship. (I'll discuss that in a later blog entry.)
"I write fiction and nonfiction," Marty wrote. "Should I seek two agents?"
A few writers have more than one agent, but it's rare. Most agents want exclusive representation, but it's more significant than that.
Previously I wrote about career planning. Becoming a well-known writer or a best-selling one or a famous writer isn't easy and it takes committed dedication. Think of it this way: After you've published your first book, you begin to attract an audience for your type of writing. Each time you publish, in theory anyway, you widen your audience. One agent said, "I expect it to take four books until my authors sell big."
Most readers, however, don't follow authors just because they're authors. They follow them within a specific genre. Fiction readers rarely turn to nonfiction and vice versa. (We writers are strange creatures so we may be an exception.)
It's extremely difficult to build name recognition in any field, but to try to sell both fiction and nonfiction makes it even more difficult. That implies writers work double shifts of writing and promoting. It rarely works.
Agents like an author who works in one genre
and builds an audience.