How long each set takes will vary, and to be honest, I've never timed it. I try to lift with 4,1,2 tempo - meaning, a count of 4 on the way up, hold for a count of one at peak and a count of 2 on the way down. When I want to particularly wreck a muscle group I will reverse that and make the eccentric portion (the negative or downward part) last the 4 seconds. Not something to do every time out because it really tears the muscles.

There shouldn't be anytime between reps. 4,1,2 then 4,1,2 for rep two - all the way to the last rep. At which time you'd rest for 60 seconds. After the final set of 12 reps with your second exercise you rest for 2 minutes before moving on to the next muscle grouping.

Using light weights to encourage proper form is a great idea. For many of the isolating exercises light weights are all you need. Upper body muscles are pretty small compared to lower body one and if you're like most of us when we started out, upper body strength is something to work on. You should definitely feel the effects of 4kg dumb bells.

When you're more comfortable with the exercises you may want to try the weights in the progression suggested by the book. I.E.:

Exercise 1:
12 reps at 60% of your maxium
10 reps at 70% of your maxium
8 reps at 75% of your maxium
6 reps at 80% of your maxium
12 reps at 75% of your maxium

Exercise 2:
12 reps at 75% of your maxium - failure

Your weights increase up until you reach set 4 where you do your 6 reps and then they inch back down for the two sets of 12 reps. If you can do the 12 reps with the same weight you used for the 6 then the weight you're using for your 6 reps is not heavy enough.

You might find this site helpful with demonstrations: fitsite exercise demos - I find some of the ones at exrx.net are show a little too fast to get a grasp of what is entailed.

Cheers,
Nat