So how long should I leave it before deleting friends after the last 'update' ?

I couldn't believe it when we got an update half way through a timed quest and discovered that loads of people in my friend's list hadn't updated making it very difficult to visit and request etc. Some deliberately held off I know but surely will have caught up by now? I'd just about cleaned my friend's list and got rid of the non players from last time and now I am faced with doing the same job all over again. Some really quite high level players haven't updated too - fed up with all the messing about? I suppose we'll never know. So is now the right time or will another update be along in a matter of days?
Yours fed-uply,

This thread discusses this:
http://www.mmuff.com/t642-time-to-cut-the-fat-identifying-inactive-friends