If you look at the load progression for the CoC grippers they go like this:
No1: 140
No2: 195
No3: 280
No4: 365
So, from 1 to 2 it's a 55lb increase (39%) but from 2 to 3 it's an 85lb increase (44%) and from 3 to 4 it's an 85lb increase (30%).
I found going from closing the 1 to the 2 didn't take too long but I still found I needed to be able to do 10 reps with the 1 before I could close the 2. Seeing that the %age increases are similar between successive grippers I am expecting that I will need to do at least 10 reps with the 2 in order to close the 3.
I started by doing sets of 5 reps, closing the No1 gripper as far as possible for each rep. (Bear in mind that the load is only the stated value when the gripper is closed. Up to that point the load will be lower.) Like Steve, I also did negs. Once I could do 3 or 4 full reps with the No1 I then started doing negs with the next hardest gripper. My technique was to squeee as hard as I could and also push my hand into my thigh to allow me to close it further. Then I would hold the load for several seconds before a slow release.
I am now experimenting with a Max-Stim style of training with my No2. I alternate hands after each rep and allow enough time between reps to get a good amount of my strength back. This way I am doing at least 20 reps per hand (although not fully closed on each rep). So far so good.
It's very easy to want to train your grip frequently as the total level of fatgue is so low. However, if I train more than three times a week I don't progress any faster. Also, your hands need time to toughen up after a hard session.