I tend to review in detail two weeks back, and two weeks forward. I find that by default I generally scan more than that, but those two views will keep me current. I have caught things before ("Wow, I need TICKETS for that trip!").
I have a real bias, though, about keeping my Projects list current. Invariably, the items on my calendar are "connected" in some way to the projects, and the faithful review of that inventory does keep me clean.