If the interviewer did not tell you when they expect to hire by, they made a typical employer error. Unfortunately, this common mistake causes headaches for themselves and the people they interview.
After a reasonable period of time (~ 2 weeks), it is acceptable to call the person who interviewed you to:
1) confirm that the job is still available
2) remind them that you are still interested in the position and
3) ask when they expect to fill the position.
Keep in mind that some companies do several rounds of interviews before hiring. It may be several months before they make a decision. If that is the case, it's important to keep yourself fresh in their minds with that follow up call :-)