We can't 'let's say'. There's nobody here inside the INZ offices to know how soon things go through ON AVERAGE. But I can tell you, since I've worked in offices with the rotating caseload system, that going through the allocation varies enormously and isn't even.
You will understand better if you think what the CO has to do. S/He has to verify everything each applicant has said about themself, by sending queries to third parties who can say, 'Yes, that's true,' or not, then ticking off each item as being satisfactory. If the reply does not satisfy the requirements, s/he has to contact the applicant for an explanation, or for more evidence, then that explanation or evidence has to be verified in its turn.
Okay. So the first time s/he opens your file, there is quite a lot to do. S/He will send off queries about every aspect of your claims. Then your file goes to the back. But every file won't be like that. Many of them will not contain any new information since the previous time s/he looked at them - queries have gone out, but no reply has come back - and those will immediately go to the back. Some files may contain one reply, confirming information, and that will mean a tick on the checklist, then the file can go to the back to wait for more replies. Maybe 20 or 30 files like those can get their turn for attention in an hour or an hour and a half. The cases which need a lot of time to deal with are those where the reply does not appear to show what the applicant said it would - maybe it's about work experience, and the employer's answer seems to prove that the applicant had a different role from what he claimed, or maybe the company appears not to be in a comparable work area. In such cases, the CO will have to work through the fine details of what will, or will not, meet the INZ criteria, maybe getting technical legal advice from a superior, and will also probably have to write and send out supplementary queries to the employer, and possibly also to the applicant. A case which needs a high level of technical attention aimed at working out if some aspect meets the requirements may well take up a day or two of the CO's time.