Big Red Machine, eyewall replacement cycles occur on a somewhat unpredictable basis -- we know that they occur, usually with some advance warning in the form of a secondary outer eyewall (whether observed by plane, satellite, or radar), but they can occur anywhere from every 12 or so hours to every 2 or so days, often time changing up in the middle of the game. They are most frequent with major hurricanes, though can be observed in weaker storms as well. There is still a lot that we don't know about them, however, and work is ongoing to develop a set of storms that underwent such cycles for future study towards improving our knowledge of them.
They oftentimes will mention eyewall replacement cycles in the discussions for such storms, but usually will only make minor modifications to the intensity forecast as a result of them if they think one is ongoing or about to occur. This is partially due to their unpredictable nature, such as how long it will last or where it will occur, as well as their nature in generally being temporary features leading to some reintensification afterward and the distance between intensity steps (12hr in the short term) in NHC advisories. For instance, I seem to recall an advisory on Dennis calling for a wekening to 115kt in 12hr from an initial intensity of 120kt, then back to 120kt 12hr later. That's about as far as they will go, though, given the current science and the need to stay towards the middle ground for the general public.
Current Tropical Model Output Plots
(or view them on the main page for any active Atlantic storms!)