Only when I know I need to catch up
Eugene:
I've found that processing is part of the natural rhythm of my day and that, in general, it works fine to get my Inboxes empty on an ad hoc basis as time and circumstances permit. There are two exceptions for me.
The first is when I know I will be offline for a specific reason - an all-day meeting, a trip to a conference, a vacation - when I now there will be an accumulation I will have to deal with that will require time to handle properly. In those cases, I block the one to three hours I think I'll need and turn off outside distractions until I'm back to empty.
The second is Fridays - the day I have found works best for me to do my Weekly review. I not only block out time mid-afternoon to do the actual review but also time immediately before lunch to get In empty so I do not fall into the trap of using WR time for processing.
HTH,
Marc