I fall somewhere between neuroheart and sdarrell: I start with a one-week period of grace, but if I have a neighbour who's inactive for four or five days, comes back on for a week, then goes AWOL for another four or five days, I tend to defriend them. (I have also defriended neighbours for farming until I belatedly realised that it's necessary to have untapped buildings for me to tap, so now I feel pretty miserable about that :( )
I have also considered defriending neighbours who cant be bothered to tap my graffitiable buildings, but thankfully so far my neighbours seem to be a pretty considerate bunch - and some of them are absolute gems (you know who you are!).
But as a rule of thumb, I reckon a week isnt too bad - unless of course they post that they're going to be absent, or they're a previously good neighbour and you afford them the benefit of the doubt. Which isn't a bad thing. Maybe we should have a "Neighbour Off Tap" thread for Tappers to post when they're going to be temporarily indisposed.