The problem with 'configurable' is that it == more code which == more time required.

maybe I'm just over sensitive to the idea of pulling new images every time you do anything. I guess the cost isn't that much, but for someone roaming on 3g its a lot of needless data movement.
I'm thinking I might settle on weekly, but potentially update for anyone you @reply/mention on the basis that you are probably more interested in their stuff being up to date.

Ideally I'm looking for a strategy that will minimise wasted bandwidth, not require lots of code to implement, and keep people moderately up to date with avatars.

at the moment I just look for a file which is twitter_id.jpg and if it doesn't exist then download avatar pic and store it in file of twitter_id.jpg
this was nice and simple. I'm sure I can come up with something lightweight to compromise between the above factors.