I think it is worth doing, I'm not sure if I would learn it to such details if I had to work on some project as soon as possible. I guess, I would just learn a few things I need at that moment and ignore the rest. (And of course, learn from compiler errors)
1. I believe the exam is not just to see how good you are in syntax. There is lot of code you need to read on exam, so to just learn answers is not so easy. Of corse there are some common problems you can prepare for, but each questions was unique to me.
Try some (free) mock exams, inquisition or any other GUI, so you can get the feeling of real exam.
2. associate - I'm not sure what is this about, perhaps Java for managment, some basic overview.
Programmer - Took me 2 months from 0 to 94%, but I really put a lot of my free time to it. Sun recommeds 6 months of preparation. I have few years experience in C++, so I guess I'm quite average.
Developer - well, you can find some topics at
SCJD forum, where it is common to spend year or two on SCJD.
SCBCD and SCWCD - I guess, that it should be possible in 6-12 months.
3. Depends on how you would like to prepare. I used K&B book. I think real world projects are not required to pass
SCJA or
SCJP. But it surely helps for SCJD and others.
4. It depends on project you plan to develop.
I (as employee) am considering just opportunity to do certification as bonus. And motivation is also that, noone wants to fail. (If my colleage can pass, I can surely too).
You as "employer" and team leader will have information, that your developers have certain level of knowlegde, and it also may have good impression on customer if you present your company as team, that has "as much" certifications, so that perhaps know what they are doing.
(Looks like I should work for Sun bussiness depart.)