Too late for this interview, but when you are interviewing it is okay to ask about their decision-making timeline.
Agree with Pragma that having another offer in hand makes a difference. If you can call and say, "I have an offer from U of X, but I will tell them that I am not interested if I get an offer from you. When will decisions be made?" that is totally understandable. Otherwise you seem impatient. Without another offer it doesn't really change anything if they tell you this week or next week (although it makes you miserable waiting.)
In general, it can vary a lot school to school and even lab to lab Sometimes they will be waiting to issue offers until they interview someone that is not interviewing with the rest of the group. They are coordinating a lot of information, interviewees, and members of the selection committee, so these things always take longer than they probably should.
Good luck during this stressful time!
Dr. E