IT Employment

I am beginning to consider starting a family. I am the sole network administrator/systems support for an accounting firm with staff of 40 and The firm includes a computer consulting group and the person who formerly had my position is a part of it. My question is: What length of maternity leave would be acceptable to ask for? Since the firm is small, FMLA is not applicable and there is no policy on the books. I fully intend to come back after the birth and am open to being on call or working part time for a length of time. Thanks.