At the end of an i1Display2 calibration*, it allows you to see "before and after". The two states are often very different - eg very cool vs quite warm, or whatever.
Some people mistakenly think that this is comparing the previous calibration with the new one. Because the difference is so great, it makes people say "Gee, I should have calibrated sooner! It was way off!!"
Relax. What that before-and-after is showing you is the new monitor profile vs the completely unprofiled state. It's not comparing last month's calibration with this month's calibration.
The truth is, modern LCD monitors don't drift much, and there will be very little (if any) visible difference between monitor profiles from month to month. If you accidentally let your calibration go for two or three months, you shouldn't be doing your images much damage, if any. In the old days of CRTs, it was a different story - those things drifted like rafts.
However, I hasten to add - if you're doing work that requires a critical level of colour accuracy, you'd be careless not to calibrate quite frequently, just to be sure.
*I don't know if other calibrators (eg Spyders) offer a similar before-and-after view. If anybody knows, I'd love to hear their experiences.