An IROP (IRegular OPeration) situation is what forces an airline to swap tails or types, and this can happen at any time. I've seen some IROPs that are managed before the first passenger comes on, others that require a quick tail swap and result in a delay, yet others that affect the next months' plan resulting in a lot of airplanes and crews being out of position.
Irregular Operations are fairly self-explanatory - they are anything that stops an aircraft or its crew from completing the flight on time in the aircraft tail assigned to the flight. This could be non-scheduled maintenance, airspace, weather; really, anything that disrupts normal operations. I've never worked for an airline that publically published how often a flight was swapped because that's not what most people care about. They want to know how often the flight actually leaves on time and they couldn't care less about what type of airplane it is. I mean, we've all heard someone say something along the lines of "I'm flying on a BoBusardier E-Jet" or something similar.
With that said and thinking over my last year, I'd have to say I've swapped tails or types on maybe 5 to 10% of my flights. Sometimes it seems to happen all in a row while on the flip side I've gone months without an IROP.