20 months post op and I still do it a few times a week. Lately, I can almost fill that thing up. I think that I lost a lot of flexibility in my ribs after two surgeries and I should have been more aggressive in combating that early on.
I wasn't. I did it routinely when I first came home but not so regularly after the first couple weeks and after 4 weeks when I was given the ok to increase my exercise I stopped completely.
The purpose of incentive spirometer is to prevent the fluid buildup in lungs. So you excersize with it as long as you have fluids in your lungs, which is different from patient to patient. I suggest to ask your doctor, if you can.
It is to fight atelectasis (collapse of lungs) and accumulation of bronchial secretions, according to this site: http://www.ceu.org/cecourses/981130/ch7.htm . I remember I was sent to X-Rays to check if my lungs are fully expanded or not yet.