The CO (or COs, when, as in your case, one leaves and another takes over) has to verify all your evidence, which means asking a third party who is in a position to know if it is true. Because it is impossible to say how urgently these other people will answer, how soon before s/he gets all the replies is unpredictable. The CO will also check out your employer - the financial soundness of the company, and their record of keeping good work practices in line with employment law.
At the same time as this, your CO will be working the other cases of their allocation (around 90 - 100) in rotation. That means, as soon as s/he has sent out the queries on your case, your file goes to the back of the queue, and s/he takes the next file, sees if there are any replies on that one that s/he can take action on, puts that one to the back, then the next, and the next, and so on. Meanwhile, any replies that come about your case will go into the file to wait till it gets to the front again for its next turn for attention.
Whenever it happens that your file comes to the top, and all the necessary replies are back, so the CO can tick all the boxes on the checklist as being satisfactory, that is when s/he can recommend you for approval. At that point, your file will be sent for second-person checking, when another official looks over everything, to check for any errors, and to guard against any possibility of corruption. There is a queue of cases waiting for attention that your file will join and work its way up before being seen by the 2pc official. How long that queue is depends on how many cases have gone through processing from several different COs just before yours got there.
Recently, processing of Residence applications has commonly been taking 10 - 12 months. If you expect it to take a year, you may get a pleasant surprise, but at least you won't be surprised at time passing. It's normal that you don't hear from the CO for periods at a time, which are when s/he has nothing to tell you because s/he is waiting for replies, and/or s/he is working on other cases which are equally her/his responsibility.