At first I didn't like to use mine. I thought I'd do OK in time. Maybe, maybe not.
Once I got home, I developed fluid in my lungs (pleural effusion) and a nasty cough. You know how great it is to have a nagging cough in the weeks just after heart surgery, don't you? I found that if I used my spirometer more, my cough was less often. I increased my spirometer "sessions" for a few weeks, and the cough went away. Also, the pleural effusion didn't need anything stronger than over-the-counter ibuprofen.
Using the spirometer more or for longer periods of time will cause no harm. Skipping it or stopping it too soon may make your life miserable for longer than necessary.