Most agency contracts require that you allow the agent the sufficient time to sell your proposal once the agent has begun submitting it to publishers. However, most agents, if you ask, will gladly cease making any future submissions to publishers on your behalf. Any time you lose faith in your agent, you should leave. Trust is the glue between authors and agents. Good agents only want satisfied clients, anyway.
Agents can, at their option, choose to be many things to clients.
An agent is a writer's most important ally. That's something your agent negotiates for you-so be sure to tell your agent ahead of time how long you want to write the manuscript, and include this information in the proposal. Just be certain not to use anything that will appear in your book in another publication. Publishers work in seasons-that is, books are published for a Spring, Summer, Fall or Winter list, so that they coincide with the publisher's catalog, marketing pushes, and other quarterly events. So, if there's a brief period of time between when you deliver your manuscript and the next publishing season, the publisher may wait until the following season before your book is published.