When should I call back?

Soon.

At this time where job getting is critical ,new returnees to Earth from Planet College may be a little hesitant about following up on calling employers back after interviews. Following rounds of interviews sometimes potential new hires can me timid about calling the company back after an interview. Sometimes, thinking they don't want to look to hungry because it might erode their bargaining position, a candidate may delay or skip calling back entirely. This in other times might be coy and even effective.
Not now.
It is wise, smart, useful, clever and even necessary to call any company you've interviewed with and ask if the position has been filled or are you still being considered. This now is crucial. There are quite a few graduates who are trying for very few jobs.

A useful time frame for a call back is about a week after the initial interview. If you are not too clear on the details it is ok to simply call the reception and ask for Human Resources. When they answer, introduce your self and tell them you had interviewed recently for a position and you'd like to know if you are still being considered or should you continue looking elsewhere.

Right now, you really need to know, so, do this. Call back and good luck!