I've been using the timestamp of the 50th thread in my inbox as a figure of merit for how much is on my plate: the more recent it is, the more is going on. The first 50 threads tend to be things I haven't looked at; the second 50 threads in the box are generally dealt with or at least transcribed to the to-do list; and the 50 after that are really only of historical interest.
For most of the last week, the top 50 threads have been spanning a time of about 24 hours.
Right now thread number 50 has a timestamp of 7:00 this morning.