The time it takes to let you know one way or the other will vary. The person who interviewed you may be flying somewhere the next day for a 5 day pattern or they may have a few days off and not want to work during that time so may not submit a report. The people who decide who progress once all information about you is submitted may have other things going on also and it probably wont be just one person. It is just the way it is at CX for a lot of things, many people who are in positions of power usually get to decide these things.
For any upgrade, SO to JFO, JFO to FO, or FO to CN, a group of guys get together and have a look over your past history, reports filed, simulator grades, anything on your permanent file, any crm issues (usually have you annoyed the wrong person), etc. This happens usually once a month when they can all get together. It would not surprise me if the same thing goes for the interview. They probably get together once a month to decide on the past months candidates. So if you interview a couple of days before they get together you will know very soon the outcome. If you interview the day after this meeting it will be a month before you hear. The guys/girls in the office are nice people so as soon as they have the information to email candidates they will do that as know it is hard waiting.
If there are other issues it may take longer, medical for example and they want clarification so it has to go through medical department. I know when I joined they used to get US confirmation that you could get a crew visa prior to a job offer.
It is all variable.