As soon as my scale numbers change or something new happens. I typically weigh myself once a week, but I haven't been moving much so I do it whenever. I never move my weight score backwards, only forwards!
2013:80.9kgs - 64.5kgs. 2014, 3 month weigh-in: Jan -Mar 62kgs March, Day 445 60kgs, GOAL achieved!
Running distance goals: 4.4km (30min) achieved 7 May 2013. 6 km (45min) 4 September 2013. 10 km (1hr 12min) achieved achieved 18 October 2013.
Competitions: Round the Bridges, Nov 17 2013 completed, 44min 07secs.
I also update every week on my weigh in day. This past Wednesday I had to move it back up a pound and it sucked but watching the ticker get closer and closer to goal is a huge motivator for me, so I gotta be honest with myself and 'earn' it or it really bothers me.