I've closed a the T, 1, 2, 3, HG250 and 300, and a PDA 280. I used a set for the grippers close to the 3. That being said it took about a month for me to get the 2 closed and I'm not sure how long to get the 3. I'd imagine it'd be much faster if I trained for it but I'd just pick a gripper up every couple weeks and give it a squeeze.
Quite a few people have closed the 4.
www.gripboard.com if you're more interested in a real grip program.